feat(gitea): default-branch lru cache

Shared LRU avoids repeated Gitea calls for default-branch resolution;
the simple stdlib map alternative would race on concurrent access without
a mutex per entry, which is more code than the LRU.

internal/tools/file_read.go

@@ -57,11 +57,11 @@ func (t *FileRead) Call(ctx context.Context, raw json.RawMessage) (json.RawMessa
 
 	ref := args.Ref
 	if ref == "" {
-		repo, err := t.c.GetRepo(ctx, args.Owner, args.Name)
+		var err error
+		ref, err = t.c.DefaultBranch(ctx, args.Owner, args.Name)
 		if err != nil {
 			return nil, err
 		}
-		ref = repo.DefaultBranch
 	}
 
 	fc, err := t.c.GetFileContents(ctx, args.Owner, args.Name, args.Path, ref)