feat(tools): repo_tree, repo_topics_update, file_read dir fix (#14, #15, #18)

repo_tree: GET /git/trees/{ref}?recursive=1 — full recursive file tree
repo_topics_update: PUT /repos/{owner}/{repo}/topics — replace topic list
file_read: detect array response and return descriptive error for dir paths

internal/gitea/files.go

@@ -27,6 +27,10 @@ func (c *Client) GetFileContents(ctx context.Context, owner, repo, path, ref str
 	if err := MapStatus(status, body); err != nil {
 		return nil, err
 	}
+	// Array response means path is a directory — guide caller to dir_list.
+	if len(body) > 0 && body[0] == '[' {
+		return nil, fmt.Errorf("%w: path %q is a directory, not a file — use dir_list", ErrValidation, path)
+	}
 	var fc FileContents
 	if err := json.Unmarshal(body, &fc); err != nil {
 		return nil, err