feat(repo): add rename_branch tool (#245)

Adds a native `rename_branch` write tool so MCP clients can rename branches without emulating it via create + delete.

Changes:
- Add `rename_branch` to the repository branch tools with `owner`, `repo`, `branch`, and `new_name` parameters.
- Call the Gitea SDK's `RenameRepoBranch` operation and propagate API errors.
- Add handler/registration tests and README tool table entries.

Closes https://gitea.com/gitea/gitea-mcp/issues/236

Assisted by Codet

Reviewed-on: https://gitea.com/gitea/gitea-mcp/pulls/245
Reviewed-by: silverwind <2021+silverwind@noreply.gitea.com>

README.md

@@ -180,6 +180,7 @@ Once configured, try `list all my repositories` in the chat box.
 | create_branch                | branch       | Write  | Create a new branch |
 | delete_branch                | branch       | Write  | Delete a branch |
 | list_branches                | branch       | Read   | List repository branches |
+| rename_branch                | branch       | Write  | Rename a branch |
 | create_tag                   | tag          | Write  | Create a tag |
 | delete_tag                   | tag          | Write  | Delete a tag |
 | get_tag                      | tag          | Read   | Get tag details |