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>

src/aegis_gitea_mcp/mcp_protocol.py

@@ -392,6 +392,40 @@ AVAILABLE_TOOLS: list[MCPTool] = [
         },
         write_operation=True,
     ),
+    _tool(
+        "update_label",
+        "Update an existing repository label, located by its current name (write-mode only).",
+        {
+            "type": "object",
+            "properties": {
+                "owner": {"type": "string"},
+                "repo": {"type": "string"},
+                "name": {"type": "string", "description": "Current label name"},
+                "new_name": {"type": "string"},
+                "color": {"type": "string", "description": "Hex color, e.g. #00aabb"},
+                "description": {"type": "string"},
+            },
+            "required": ["owner", "repo", "name"],
+            "additionalProperties": False,
+        },
+        write_operation=True,
+    ),
+    _tool(
+        "remove_labels",
+        "Remove labels (by name) from an issue or pull request (write-mode only).",
+        {
+            "type": "object",
+            "properties": {
+                "owner": {"type": "string"},
+                "repo": {"type": "string"},
+                "issue_number": {"type": "integer", "minimum": 1},
+                "labels": {"type": "array", "items": {"type": "string"}, "minItems": 1},
+            },
+            "required": ["owner", "repo", "issue_number", "labels"],
+            "additionalProperties": False,
+        },
+        write_operation=True,
+    ),
 ]