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>

docs/api-reference.md

@@ -67,6 +67,7 @@ Scope requirements:
 - `create_pr_comment` (`owner`, `repo`, `pull_number`, `body`)
 - `add_labels` (`owner`, `repo`, `issue_number`, `labels`)
 - `assign_issue` (`owner`, `repo`, `issue_number`, `assignees`)
+- `create_label` (`owner`, `repo`, `name`, `color` hex e.g. `#00aabb`, optional `description`, `exclusive`)
 
 ## Validation and Limits