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/tools/arguments.py

@@ -220,6 +220,16 @@ class AssignIssueArgs(RepositoryArgs):
     assignees: list[str] = Field(..., min_length=1, max_length=20)
 
 
+class CreateLabelArgs(RepositoryArgs):
+    """Arguments for create_label."""
+
+    name: str = Field(..., min_length=1, max_length=50)
+    # Gitea requires a hex color; accept it with or without a leading '#'.
+    color: str = Field(..., pattern=r"^#?[0-9A-Fa-f]{6}$")
+    description: str = Field(default="", max_length=1000)
+    exclusive: bool = Field(default=False)
+
+
 def extract_repository(arguments: dict[str, object]) -> str | None:
     """Extract `owner/repo` from raw argument mapping.