feat(connector): add search and fetch tools for ChatGPT connectors

Add a new "connector" domain exposing exactly-named "search" and "fetch"
MCP tools for ChatGPT-style connector compatibility (gitea#84). search
looks up repositories via the existing SearchRepos SDK call, optionally
resolving an owner/org name to an owner ID, and returns concise entries
(id, name, url, html_url). fetch reuses GetContents to read a file and
decodes its base64 content, returning path, sha, size, encoding and the
decoded content. Registers connector.Tool in operation.go and documents
the two tools in all three README tables.

Co-Authored-By: Codet <codet@commitgo.dev> (GPT-5-Codex)

README.zh-cn.md

@@ -191,6 +191,8 @@ Cursor 等客户端可使用 stdio 命令：
 | get_release                  | release      | 读取 | 按 ID 获取版本发布 |
 | get_latest_release           | release      | 读取 | 获取最新版本发布 |
 | list_releases                | release      | 读取 | 列出仓库版本发布 |
+| search                       | connector    | 读取 | 为 ChatGPT 风格的连接器按关键字搜索 Gitea 仓库 |
+| fetch                        | connector    | 读取 | 为 ChatGPT 风格的连接器获取文件的解码内容与元数据 |
 
 > **说明：** 部分工具是聚合的、基于操作的工具，单个工具通过 `method` 参数暴露多个操作。当服务器以只读模式运行时（`-r` / `GITEA_READONLY`），访问为「写入」的工具会被隐藏；可通过 `-S` / `--scope`（`GITEA_SCOPES`）按范围过滤，或通过 `-O` / `--tools`（`GITEA_TOOLS`）按工具名称过滤对外暴露的工具集合。