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/server.py

@@ -83,6 +83,7 @@ from aegis_gitea_mcp.tools.write_tools import (
     assign_issue_tool,
     create_issue_comment_tool,
     create_issue_tool,
+    create_label_tool,
     create_pr_comment_tool,
     update_issue_tool,
 )
@@ -342,6 +343,7 @@ TOOL_HANDLERS: dict[str, ToolHandler] = {
     "create_pr_comment": create_pr_comment_tool,
     "add_labels": add_labels_tool,
     "assign_issue": assign_issue_tool,
+    "create_label": create_label_tool,
 }