Resolves the long-standing problem that label tools passed names while Gitea's
API requires numeric label ids.
- gitea_client: add _resolve_label_ids() helper; create_issue and add_labels now
resolve label names to ids (case-insensitive) and raise a clear "Unknown
label(s)" error instead of a generic 500.
- New tools: remove_labels (by name) and update_label (located by current name).
- Register both write tools and document the name-based label contract.
- Tests: resolver mapping + unknown-label error, add_labels id translation,
update_label and remove_labels handlers.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>