style: use fmt.Fprintf instead of WriteString(fmt.Sprintf) (staticcheck QF1012)

Mechanical fix applied by golangci-lint --fix. Pre-existing debt that fails the staticcheck QF1012 quickfix check; no behavior change. Clears the lint gate so subsequent commits in this package set pass.

cmd/docgen/main.go

@@ -211,7 +211,7 @@ func GenerateMarkdown() string {
 	var sb strings.Builder
 
 	sb.WriteString("# MCP Filepuff API Reference\n\n")
-	sb.WriteString(fmt.Sprintf("> Auto-generated on %s\n\n", time.Now().Format("2006-01-02")))
+	fmt.Fprintf(&sb, "> Auto-generated on %s\n\n", time.Now().Format("2006-01-02"))
 	sb.WriteString("This document provides detailed API documentation for all MCP tools available in filepuff.\n\n")
 
 	// Table of Contents
@@ -228,9 +228,9 @@ func GenerateMarkdown() string {
 	}
 
 	for _, cat := range categoryOrder {
-		sb.WriteString(fmt.Sprintf("### %s\n", cat))
+		fmt.Fprintf(&sb, "### %s\n", cat)
 		for _, tool := range categories[cat] {
-			sb.WriteString(fmt.Sprintf("- [`%s`](#%s)\n", tool.Name, tool.Name))
+			fmt.Fprintf(&sb, "- [`%s`](#%s)\n", tool.Name, tool.Name)
 		}
 		sb.WriteString("\n")
 	}
@@ -240,11 +240,11 @@ func GenerateMarkdown() string {
 	sb.WriteString("## Tool Reference\n\n")
 
 	for _, cat := range categoryOrder {
-		sb.WriteString(fmt.Sprintf("### %s\n\n", cat))
+		fmt.Fprintf(&sb, "### %s\n\n", cat)
 
 		for _, tool := range categories[cat] {
-			sb.WriteString(fmt.Sprintf("#### `%s`\n\n", tool.Name))
-			sb.WriteString(fmt.Sprintf("%s\n\n", tool.Description))
+			fmt.Fprintf(&sb, "#### `%s`\n\n", tool.Name)
+			fmt.Fprintf(&sb, "%s\n\n", tool.Description)
 
 			// Read-only indicator
 			if tool.ReadOnly {
@@ -263,8 +263,8 @@ func GenerateMarkdown() string {
 					if param.Required {
 						required = "**Yes**"
 					}
-					sb.WriteString(fmt.Sprintf("| `%s` | `%s` | %s | %s |\n",
-						param.Name, param.Type, required, param.Description))
+					fmt.Fprintf(&sb, "| `%s` | `%s` | %s | %s |\n",
+						param.Name, param.Type, required, param.Description)
 				}
 				sb.WriteString("\n")
 			} else {
@@ -285,7 +285,7 @@ func GenerateMarkdown() string {
 			if len(tool.Notes) > 0 {
 				sb.WriteString("**Notes:**\n\n")
 				for _, note := range tool.Notes {
-					sb.WriteString(fmt.Sprintf("- %s\n", note))
+					fmt.Fprintf(&sb, "- %s\n", note)
 				}
 				sb.WriteString("\n")
 			}

internal/metrics/metrics.go

@@ -278,27 +278,27 @@ func (r *Registry) Expose() string {
 	// Export counters
 	for _, c := range r.counters {
 		if c.help != "" {
-			sb.WriteString(fmt.Sprintf("# HELP %s %s\n", c.name, c.help))
+			fmt.Fprintf(&sb, "# HELP %s %s\n", c.name, c.help)
 		}
-		sb.WriteString(fmt.Sprintf("# TYPE %s counter\n", c.name))
-		sb.WriteString(fmt.Sprintf("%s%s %d\n", c.name, formatLabels(c.labels), c.value.Load()))
+		fmt.Fprintf(&sb, "# TYPE %s counter\n", c.name)
+		fmt.Fprintf(&sb, "%s%s %d\n", c.name, formatLabels(c.labels), c.value.Load())
 	}
 
 	// Export gauges
 	for _, g := range r.gauges {
 		if g.help != "" {
-			sb.WriteString(fmt.Sprintf("# HELP %s %s\n", g.name, g.help))
+			fmt.Fprintf(&sb, "# HELP %s %s\n", g.name, g.help)
 		}
-		sb.WriteString(fmt.Sprintf("# TYPE %s gauge\n", g.name))
-		sb.WriteString(fmt.Sprintf("%s%s %d\n", g.name, formatLabels(g.labels), g.value.Load()))
+		fmt.Fprintf(&sb, "# TYPE %s gauge\n", g.name)
+		fmt.Fprintf(&sb, "%s%s %d\n", g.name, formatLabels(g.labels), g.value.Load())
 	}
 
 	// Export histograms
 	for _, h := range r.histograms {
 		if h.help != "" {
-			sb.WriteString(fmt.Sprintf("# HELP %s %s\n", h.name, h.help))
+			fmt.Fprintf(&sb, "# HELP %s %s\n", h.name, h.help)
 		}
-		sb.WriteString(fmt.Sprintf("# TYPE %s histogram\n", h.name))
+		fmt.Fprintf(&sb, "# TYPE %s histogram\n", h.name)
 
 		// Cumulative bucket counts
 		var cumulative int64
@@ -306,26 +306,26 @@ func (r *Registry) Expose() string {
 			cumulative += h.counts[i].Load()
 			labelStr := formatLabels(h.labels)
 			if labelStr == "" {
-				sb.WriteString(fmt.Sprintf("%s_bucket{le=\"%g\"} %d\n", h.name, bound, cumulative))
+				fmt.Fprintf(&sb, "%s_bucket{le=\"%g\"} %d\n", h.name, bound, cumulative)
 			} else {
 				// Insert le label into existing labels
-				sb.WriteString(fmt.Sprintf("%s_bucket%s %d\n", h.name,
-					strings.Replace(labelStr, "}", fmt.Sprintf(",le=\"%g\"}", bound), 1), cumulative))
+				fmt.Fprintf(&sb, "%s_bucket%s %d\n", h.name,
+					strings.Replace(labelStr, "}", fmt.Sprintf(",le=\"%g\"}", bound), 1), cumulative)
 			}
 		}
 		// +Inf bucket
 		cumulative += h.counts[len(h.buckets)].Load()
 		labelStr := formatLabels(h.labels)
 		if labelStr == "" {
-			sb.WriteString(fmt.Sprintf("%s_bucket{le=\"+Inf\"} %d\n", h.name, cumulative))
+			fmt.Fprintf(&sb, "%s_bucket{le=\"+Inf\"} %d\n", h.name, cumulative)
 		} else {
-			sb.WriteString(fmt.Sprintf("%s_bucket%s %d\n", h.name,
-				strings.Replace(labelStr, "}", ",le=\"+Inf\"}", 1), cumulative))
+			fmt.Fprintf(&sb, "%s_bucket%s %d\n", h.name,
+				strings.Replace(labelStr, "}", ",le=\"+Inf\"}", 1), cumulative)
 		}
 
 		// Sum and count
-		sb.WriteString(fmt.Sprintf("%s_sum%s %g\n", h.name, formatLabels(h.labels), h.Sum()))
-		sb.WriteString(fmt.Sprintf("%s_count%s %d\n", h.name, formatLabels(h.labels), h.count.Load()))
+		fmt.Fprintf(&sb, "%s_sum%s %g\n", h.name, formatLabels(h.labels), h.Sum())
+		fmt.Fprintf(&sb, "%s_count%s %d\n", h.name, formatLabels(h.labels), h.count.Load())
 	}
 
 	return sb.String()

internal/parser/skeleton.go

@@ -44,11 +44,11 @@ func SkeletonFile(ctx context.Context, reg *Registry, filename string, content [
 func renderSymbolsOnly(syms []protocol.Symbol, _ string, lang protocol.Language, content []byte) string {
 	lines := strings.Split(string(content), "\n")
 	var sb strings.Builder
-	sb.WriteString(fmt.Sprintf("// skeleton unavailable for %s — symbol list only\n", lang))
+	fmt.Fprintf(&sb, "// skeleton unavailable for %s — symbol list only\n", lang)
 	for _, s := range syms {
-		sb.WriteString(fmt.Sprintf("// %s %s  (line %d)\n", s.Kind, s.Name, s.Location.Line))
+		fmt.Fprintf(&sb, "// %s %s  (line %d)\n", s.Kind, s.Name, s.Location.Line)
 		if s.Doc != "" {
-			sb.WriteString(fmt.Sprintf("// doc: %s\n", s.Doc))
+			fmt.Fprintf(&sb, "// doc: %s\n", s.Doc)
 		}
 		if s.Location.Line >= 1 && s.Location.Line <= len(lines) {
 			sb.WriteString(lines[s.Location.Line-1] + " { ... }\n")

internal/query/query.go

@@ -486,7 +486,7 @@ func FormatResultsWithOptions(results []MatchResult, maxResults int, format stri
 	// Emit preamble only when verbose=true is explicitly passed (opt-in, default off).
 	wantVerbose := len(verbose) > 0 && verbose[0]
 	if wantVerbose {
-		sb.WriteString(fmt.Sprintf("Found %d match(es):\n", renderCount))
+		fmt.Fprintf(&sb, "Found %d match(es):\n", renderCount)
 	}
 
 	switch format {
@@ -498,13 +498,13 @@ func FormatResultsWithOptions(results []MatchResult, maxResults int, format stri
 				nodeType = r.Node.Type()
 			}
 			firstLine := firstLineOf(r.Text, 80)
-			sb.WriteString(fmt.Sprintf("%s:%d (%s) %s\n", r.File, r.Location.Line, nodeType, firstLine))
+			fmt.Fprintf(&sb, "%s:%d (%s) %s\n", r.File, r.Location.Line, nodeType, firstLine)
 		}
 
 	case "location":
 		for i := 0; i < renderCount; i++ {
 			r := results[i]
-			sb.WriteString(fmt.Sprintf("%s:%d\n", r.File, r.Location.Line))
+			fmt.Fprintf(&sb, "%s:%d\n", r.File, r.Location.Line)
 		}
 
 	default: // "verbose"
@@ -514,7 +514,7 @@ func FormatResultsWithOptions(results []MatchResult, maxResults int, format stri
 			if r.Node != nil {
 				nodeType = r.Node.Type()
 			}
-			sb.WriteString(fmt.Sprintf("**%s:%d** (%s)\n", r.File, r.Location.Line, nodeType))
+			fmt.Fprintf(&sb, "**%s:%d** (%s)\n", r.File, r.Location.Line, nodeType)
 
 			// Truncate very long text
 			text := r.Text
@@ -538,7 +538,7 @@ func FormatResultsWithOptions(results []MatchResult, maxResults int, format stri
 					if len(capText) > 50 {
 						capText = capText[:50] + "..."
 					}
-					sb.WriteString(fmt.Sprintf("$%s=%s", name, capText))
+					fmt.Fprintf(&sb, "$%s=%s", name, capText)
 				}
 				sb.WriteString("\n")
 			}
@@ -549,7 +549,7 @@ func FormatResultsWithOptions(results []MatchResult, maxResults int, format stri
 	if remaining > 0 {
 		// Caller must embed the cursor token; we just append the remaining count hint.
 		// The actual [cursor: ...] line is written by the handler after calling MakeCursor.
-		sb.WriteString(fmt.Sprintf("[remaining: %d]\n", remaining))
+		fmt.Fprintf(&sb, "[remaining: %d]\n", remaining)
 	}
 
 	return sb.String()

internal/search/search.go

@@ -389,13 +389,13 @@ func (s *Searcher) FormatResultsWithOptions(results *SearchResults, opts FormatO
 	if opts.Verbose {
 		totalMatches := len(results.Results)
 		fileCount := len(fileResults)
-		sb.WriteString(fmt.Sprintf("Found %d matches in %d files", totalMatches, fileCount))
+		fmt.Fprintf(&sb, "Found %d matches in %d files", totalMatches, fileCount)
 		if results.Truncated {
-			sb.WriteString(fmt.Sprintf(" (truncated, total: %d)", results.Total))
+			fmt.Fprintf(&sb, " (truncated, total: %d)", results.Total)
 		}
 		sb.WriteString(":\n\n")
 	} else if results.Truncated {
-		sb.WriteString(fmt.Sprintf("(truncated, showing subset of %d total matches)\n\n", results.Total))
+		fmt.Fprintf(&sb, "(truncated, showing subset of %d total matches)\n\n", results.Total)
 	}
 
 	// Write results grouped by file
@@ -408,7 +408,7 @@ func (s *Searcher) FormatResultsWithOptions(results *SearchResults, opts FormatO
 			}
 		}
 
-		sb.WriteString(fmt.Sprintf("**%s**\n", relPath))
+		fmt.Fprintf(&sb, "**%s**\n", relPath)
 
 		if opts.Cluster {
 			writeClusteredResults(&sb, fileResults[file])

internal/server/session_integration_test.go

@@ -199,7 +199,7 @@ func TestSessionPrefsASTQueryMaxResults(t *testing.T) {
 	var sb strings.Builder
 	sb.WriteString("package main\n\n")
 	for i := 0; i < 5; i++ {
-		sb.WriteString(fmt.Sprintf("func Fn%c() {}\n\n", rune('A'+i)))
+		fmt.Fprintf(&sb, "func Fn%c() {}\n\n", rune('A'+i))
 	}
 	testFile := filepath.Join(tmpDir, "many.go")
 	if err := os.WriteFile(testFile, []byte(sb.String()), 0600); err != nil {
@@ -281,7 +281,7 @@ func TestSessionPrefsMaxResultsExplicitOverride(t *testing.T) {
 	var sb strings.Builder
 	sb.WriteString("package main\n\n")
 	for i := 0; i < 5; i++ {
-		sb.WriteString(fmt.Sprintf("func Fn%c() {}\n\n", rune('A'+i)))
+		fmt.Fprintf(&sb, "func Fn%c() {}\n\n", rune('A'+i))
 	}
 	testFile := filepath.Join(tmpDir, "many.go")
 	if err := os.WriteFile(testFile, []byte(sb.String()), 0600); err != nil {

pkg/errors/errors.go

@@ -77,24 +77,24 @@ func (e *StructuredError) Error() string {
 	var sb strings.Builder
 
 	// Error code and message
-	sb.WriteString(fmt.Sprintf("[%d] %s", e.Code, e.Message))
+	fmt.Fprintf(&sb, "[%d] %s", e.Code, e.Message)
 
 	// Context if available
 	if len(e.Context) > 0 {
 		sb.WriteString("\nContext:")
 		for k, v := range e.Context {
-			sb.WriteString(fmt.Sprintf("\n  %s: %v", k, v))
+			fmt.Fprintf(&sb, "\n  %s: %v", k, v)
 		}
 	}
 
 	// Remediation if available
 	if e.Remediation != "" {
-		sb.WriteString(fmt.Sprintf("\nHow to fix: %s", e.Remediation))
+		fmt.Fprintf(&sb, "\nHow to fix: %s", e.Remediation)
 	}
 
 	// Underlying cause if available
 	if e.Cause != nil {
-		sb.WriteString(fmt.Sprintf("\nCaused by: %v", e.Cause))
+		fmt.Fprintf(&sb, "\nCaused by: %v", e.Cause)
 	}
 
 	return sb.String()
@@ -215,7 +215,7 @@ func captureStack(skip int) string {
 	for {
 		frame, more := frames.Next()
 		if !strings.Contains(frame.File, "runtime/") {
-			sb.WriteString(fmt.Sprintf("\n  %s:%d %s", frame.File, frame.Line, frame.Function))
+			fmt.Fprintf(&sb, "\n  %s:%d %s", frame.File, frame.Line, frame.Function)
 		}
 		if !more {
 			break