refactor: remove dead code, deduplicate defaultHome, simplify mcpEntry + kilo render

Dead code removed:
- BuildProviders, buildModels, ProviderEntry (no production callers)
- ZedLanguageModels (shadowed by adapter-local version, wrong schema)
- GatewayProviderID deprecated constant (zero references)
- Corresponding dead tests for BuildProviders

Deduplication:
- Extract common.DefaultHome() to internal/adapter/common/home.go
- All 7 adapters call common.DefaultHome() instead of local copies (-49 lines)

Simplification:
- mcp.go: merge MCPClaudeStyle/MCPCrushStyle branches (identical URL path,
  one-line difference in stdio path) — clearer intent, fewer lines
- kilo.go: extract opencode.jsonc merge to mergeOpenCodeJSONC() helper,
  Render() drops from 92 to ~55 lines
- goose.go: strings.IndexByte → strings.Cut in splitProviderModel

internal/adapter/common/home.go

@@ -0,0 +1,12 @@
+package common
+
+import "os"
+
+// DefaultHome returns the current user's home directory, or "" on error.
+func DefaultHome() string {
+	h, err := os.UserHomeDir()
+	if err != nil {
+		return ""
+	}
+	return h
+}