V2/token optimization (#11)

* v2.0: token-optimization overhaul

Additive (backward-compatible flags):
- file_read: skeleton mode, strip (imports/license/block_comments),
  compact_line_numbers, 8-char etag with prefix-match compat
- ast_query: format=verbose|compact|location, pagination cursor
- file_search: cluster mode, pagination cursor
- lsp_query (references): compact output

Breaking (v2):
- Preambles removed; opt-in verbose=true restores
- edit_apply: response=count|diff|none, default count
- ping tool removed
- symbol_at/find_definition/find_references merged into lsp_query
- Tool descriptions trimmed -83%, help moved to filepuff://help/<tool>
- Batch file_read dedups by etag

Protocol:
- ResourceLink returned for file_read >64 KiB (force_inline override)
- OnAfterInitialize hook reads capabilities.experimental.filepuff
  for session defaults (default_format, default_max_results,
  default_cluster, compact_refs, line_numbers,
  resource_link_threshold)

* fix: drop --max-total-count from ripgrep args

The flag does not exist in stable ripgrep (confirmed up to 15.1.0 --
"unrecognized flag --max-total-count, similar flags that are
available: --max-count"). Every file_search call failed on hosts with
stock rg. --max-count is per-file, not a drop-in replacement, so rely
on the in-process truncation in parseOutput that was already the
documented safety net.

internal/server/session.go

@@ -0,0 +1,145 @@
+// Package server implements the MCP server for file operations.
+package server
+
+import (
+	"sync/atomic"
+	"unsafe"
+)
+
+// SessionPrefs holds client-declared session-wide preferences parsed from
+// InitializeRequest.Params.Capabilities.Experimental["filepuff"].
+//
+// These act as defaults; explicit per-call flags always override them.
+//
+// Supported keys in the "filepuff" experimental map:
+//
+//	terse                  bool   — no-op (v2 default is already terse; reserved for future)
+//	default_format         string — ast_query format default ("verbose"|"compact"|"location")
+//	default_max_results    int    — file_search and ast_query max_results when not supplied
+//	default_cluster        bool   — file_search cluster default
+//	compact_refs           bool   — lsp_query references compact default
+//	line_numbers           string — file_read line prefix default ("none"|"compact"|"full")
+//	resource_link_threshold int   — per-session override for cfg.ResourceLinkThresholdBytes
+type SessionPrefs struct {
+	// ASTQueryFormat is the default format for ast_query ("verbose", "compact", "location").
+	// Empty string means "use handler built-in default".
+	ASTQueryFormat string
+
+	// DefaultMaxResults is the default max_results for file_search and ast_query.
+	// 0 means "use handler built-in default".
+	DefaultMaxResults int
+
+	// DefaultCluster is the default cluster flag for file_search.
+	// nil means "use handler built-in default (false)".
+	DefaultCluster *bool
+
+	// CompactRefs is the default compact flag for lsp_query action=references.
+	// nil means "use handler built-in default (false)".
+	CompactRefs *bool
+
+	// LineNumbers controls the file_read line prefix default.
+	// "" = use handler built-in default (full).
+	// "none" = no line numbers.
+	// "compact" = compact prefix (N│content).
+	// "full" = standard padded prefix (  N│ content).
+	LineNumbers string
+
+	// ResourceLinkThreshold overrides cfg.ResourceLinkThresholdBytes for this session.
+	// 0 means "use config default".
+	ResourceLinkThreshold int
+}
+
+// boolPtr returns a pointer to a bool value.
+func boolPtr(b bool) *bool { return &b }
+
+// ParseSessionPrefs parses the raw map from
+// InitializeRequest.Params.Capabilities.Experimental["filepuff"].
+// Unknown keys are silently ignored. Type mismatches for individual keys are
+// silently ignored (key is treated as absent). Returns zero-value SessionPrefs
+// when raw is nil or empty — callers should treat zero values as "use built-in defaults".
+func ParseSessionPrefs(raw map[string]any) SessionPrefs {
+	if len(raw) == 0 {
+		return SessionPrefs{}
+	}
+
+	var p SessionPrefs
+
+	if v, ok := raw["default_format"]; ok {
+		if s, ok := v.(string); ok {
+			switch s {
+			case "verbose", "compact", "location":
+				p.ASTQueryFormat = s
+			}
+		}
+	}
+
+	if v, ok := raw["default_max_results"]; ok {
+		if n := toInt(v); n > 0 {
+			p.DefaultMaxResults = n
+		}
+	}
+
+	if v, ok := raw["default_cluster"]; ok {
+		if b, ok := v.(bool); ok {
+			p.DefaultCluster = boolPtr(b)
+		}
+	}
+
+	if v, ok := raw["compact_refs"]; ok {
+		if b, ok := v.(bool); ok {
+			p.CompactRefs = boolPtr(b)
+		}
+	}
+
+	if v, ok := raw["line_numbers"]; ok {
+		if s, ok := v.(string); ok {
+			switch s {
+			case "none", "compact", "full":
+				p.LineNumbers = s
+			}
+		}
+	}
+
+	if v, ok := raw["resource_link_threshold"]; ok {
+		if n := toInt(v); n >= 0 {
+			p.ResourceLinkThreshold = n
+		}
+	}
+
+	return p
+}
+
+// toInt converts numeric JSON-decoded values (float64, int, int64) to int.
+// Returns 0 for unsupported types or negative values.
+func toInt(v any) int {
+	switch n := v.(type) {
+	case float64:
+		if n >= 0 {
+			return int(n)
+		}
+	case int:
+		if n >= 0 {
+			return n
+		}
+	case int64:
+		if n >= 0 {
+			return int(n)
+		}
+	}
+	return 0
+}
+
+// sessionPrefsPtr is an atomic pointer helper for thread-safe access to *SessionPrefs.
+// We use atomic store/load so the hook (called once at init) and handlers (many goroutines)
+// never race. The prefs are write-once after initialization.
+type sessionPrefsPtr struct {
+	p unsafe.Pointer // *SessionPrefs
+}
+
+func (sp *sessionPrefsPtr) Store(prefs *SessionPrefs) {
+	atomic.StorePointer(&sp.p, unsafe.Pointer(prefs))
+}
+
+func (sp *sessionPrefsPtr) Load() *SessionPrefs {
+	return (*SessionPrefs)(atomic.LoadPointer(&sp.p))
+}