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/mcp_protocol.py

@@ -374,6 +374,24 @@ AVAILABLE_TOOLS: list[MCPTool] = [
         },
         write_operation=True,
     ),
+    _tool(
+        "create_label",
+        "Create a repository label (write-mode only).",
+        {
+            "type": "object",
+            "properties": {
+                "owner": {"type": "string"},
+                "repo": {"type": "string"},
+                "name": {"type": "string"},
+                "color": {"type": "string", "description": "Hex color, e.g. #00aabb"},
+                "description": {"type": "string", "default": ""},
+                "exclusive": {"type": "boolean", "default": False},
+            },
+            "required": ["owner", "repo", "name", "color"],
+            "additionalProperties": False,
+        },
+        write_operation=True,
+    ),
 ]