feat: dual-mode — MCP server and CLI skill in one file

Expose the same 66 Gitea tools two ways from server/gitea_mcp.py:
- MCP server (no subcommand -> mcp.run()), as before
- CLI: `<tool> --flags` prints one JSON object {success, data/error}

A new @tool decorator registers each function in both FastMCP and a TOOLS
registry; a CLI dispatcher builds argparse from each function's signature and
wraps results in the repo's JSON+success contract. Adds `list-tools` for
discovery. No tool logic duplicated.

- Rewrite skills/gitea/SKILL.md for CLI-first usage (MCP kept as an option)
- Sync version to 0.3.0 across plugin.json, pyproject.toml, SKILL.md
- Add tests/unit/test_cli.py (dispatch routing, flag typing, errors, main)
- Document dual-mode in README.md and CLAUDE.md
- Ignore coverage artifacts

Tests: 119 passed, 93% coverage.

.gitignore

@@ -10,6 +10,12 @@ __pycache__/
 dist/
 .env
 
+# Test / coverage artifacts
+.coverage
+.coverage.*
+.pytest_cache/
+htmlcov/
+
 # Gitea API spec — large auto-generated file, download from your Gitea instance
 gitea-api.yaml