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_write_branch.go

@@ -75,11 +75,11 @@ func (t *FileWriteBranch) Call(ctx context.Context, raw json.RawMessage) (json.R
 	if !exists {
 		base := args.Base
 		if base == "" {
-			repo, err := t.c.GetRepo(ctx, args.Owner, args.Name)
+			var err error
+			base, err = t.c.DefaultBranch(ctx, args.Owner, args.Name)
 			if err != nil {
 				return nil, err
 			}
-			base = repo.DefaultBranch
 		}
 		if err := t.c.CreateBranch(ctx, args.Owner, args.Name, args.Branch, base); err != nil {
 			return nil, err