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/tools/arguments.py

@@ -230,6 +230,29 @@ class CreateLabelArgs(RepositoryArgs):
     exclusive: bool = Field(default=False)
 
 
+class UpdateLabelArgs(RepositoryArgs):
+    """Arguments for update_label (located by current name)."""
+
+    name: str = Field(..., min_length=1, max_length=50)
+    new_name: str | None = Field(default=None, min_length=1, max_length=50)
+    color: str | None = Field(default=None, pattern=r"^#?[0-9A-Fa-f]{6}$")
+    description: str | None = Field(default=None, max_length=1000)
+
+    @model_validator(mode="after")
+    def require_change(self) -> UpdateLabelArgs:
+        """Require at least one mutable field in the update payload."""
+        if self.new_name is None and self.color is None and self.description is None:
+            raise ValueError("At least one of new_name, color, or description must be provided")
+        return self
+
+
+class RemoveLabelsArgs(RepositoryArgs):
+    """Arguments for remove_labels."""
+
+    issue_number: int = Field(..., ge=1)
+    labels: list[str] = Field(..., min_length=1, max_length=20)
+
+
 def extract_repository(arguments: dict[str, object]) -> str | None:
     """Extract `owner/repo` from raw argument mapping.