feat: add create_label write tool

Adds a create_label write-mode tool so labels can be created in a repository
through the MCP server (previously there was no way to define labels, which
blocked attaching labels to issues). Follows the full tool checklist:

- arguments.py: CreateLabelArgs (name, hex color, optional description/exclusive),
  with extra=forbid and a hex-color pattern.
- gitea_client.py: create_label() POSTing to /repos/{owner}/{repo}/labels with
  url-encoded path segments.
- write_tools.py: create_label_tool handler; normalizes the color to a leading
  '#', bounds text output, and lets auth/authz errors surface.
- mcp_protocol.py: register create_label (write_operation=True).
- server.py: wire create_label into TOOL_HANDLERS.
- docs/api-reference.md: document create_label.
- tests: success path, color normalization, and invalid-color rejection.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/aegis_gitea_mcp/gitea_client.py

@@ -668,6 +668,33 @@ class GiteaClient:
         )
         return result if isinstance(result, dict) else {}
 
+    async def create_label(
+        self,
+        owner: str,
+        repo: str,
+        *,
+        name: str,
+        color: str,
+        description: str = "",
+        exclusive: bool = False,
+    ) -> dict[str, Any]:
+        """Create a repository label."""
+        payload: dict[str, Any] = {
+            "name": name,
+            "color": color,
+            "description": description,
+            "exclusive": exclusive,
+        }
+        result = await self._request(
+            "POST",
+            f"/api/v1/repos/{quote(owner, safe='')}/{quote(repo, safe='')}/labels",
+            json_body=payload,
+            correlation_id=str(
+                self.audit.log_tool_invocation(tool_name="create_label", result_status="pending")
+            ),
+        )
+        return result if isinstance(result, dict) else {}
+
     async def add_labels(
         self,
         owner: str,