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/adapters/cagent/cagent.go

@@ -29,21 +29,13 @@ func WithHome(h string) Option { return func(a *Adapter) { a.home = h } }
 
 // New returns a new Adapter with the given options applied.
 func New(opts ...Option) *Adapter {
-	a := &Adapter{home: defaultHome()}
+	a := &Adapter{home: common.DefaultHome()}
 	for _, o := range opts {
 		o(a)
 	}
 	return a
 }
 
-func defaultHome() string {
-	h, err := os.UserHomeDir()
-	if err != nil {
-		return ""
-	}
-	return h
-}
-
 // Name returns the harness identifier.
 func (a *Adapter) Name() string { return name }