feat: complete label management (name->id resolution, update/remove)

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>

tests/test_tools_extended.py

@@ -24,7 +24,9 @@ from aegis_gitea_mcp.tools.write_tools import (
     create_issue_tool,
     create_label_tool,
     create_pr_comment_tool,
+    remove_labels_tool,
     update_issue_tool,
+    update_label_tool,
 )
 
 
@@ -101,6 +103,17 @@ class StubGitea:
     async def create_label(self, owner, repo, *, name, color, description="", exclusive=False):
         return {"id": 5, "name": name, "color": color, "description": description, "url": "u"}
 
+    async def update_label(self, owner, repo, *, name, new_name=None, color=None, description=None):
+        return {
+            "id": 5,
+            "name": new_name or name,
+            "color": color or "#ffffff",
+            "description": description or "",
+        }
+
+    async def remove_labels(self, owner, repo, index, labels):
+        return []
+
 
 class ErrorGitea(StubGitea):
     """Stub that raises backend errors for failure-mode coverage."""
@@ -178,6 +191,16 @@ async def test_extended_read_tools_failure_mode() -> None:
             {"owner": "acme", "repo": "app", "name": "bug", "color": "#ff0000"},
             "id",
         ),
+        (
+            update_label_tool,
+            {"owner": "acme", "repo": "app", "name": "bug", "new_name": "defect"},
+            "id",
+        ),
+        (
+            remove_labels_tool,
+            {"owner": "acme", "repo": "app", "issue_number": 1, "labels": ["bug"]},
+            "removed",
+        ),
     ],
 )
 async def test_write_tools_success(tool, args, expected_key):