fix(mcp): map tool-not-found to CodeNotFound via registry sentinel

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/registry/registry.go

@@ -4,8 +4,11 @@ import (
 	"context"
 	"encoding/json"
 	"errors"
+	"fmt"
 )
 
+var ErrToolNotFound = errors.New("tool not found")
+
 type ToolDescriptor struct {
 	Name        string          `json:"name"`
 	Description string          `json:"description"`
@@ -36,7 +39,7 @@ func (r *Registry) Tools() []ToolDescriptor {
 func (r *Registry) Dispatch(ctx context.Context, name string, args json.RawMessage) (json.RawMessage, error) {
 	t, ok := r.tools[name]
 	if !ok {
-		return nil, errors.New("tool not found: " + name)
+		return nil, fmt.Errorf("tool %q: %w", name, ErrToolNotFound)
 	}
 	return t.Call(ctx, args)
 }