Add CLI MCP server

[yakky]

Adds an MCP (Model Context Protocol) server to python-taiga, exposing Taiga entities as tools that AI agents (e.g. Claude Code) can call directly.

• New taiga.mcp_server package built on fastmcp:
  • server.py — tools covering projects, user stories, tasks, issues, epics, milestones and wiki pages (list/get/create/update/delete as applicable), plus whoami and get_history (reads the full comment/change history of a user story, task, issue, epic or wiki page).
  • auth.py — client construction from TAIGA_HOST + TAIGA_TOKEN, or TAIGA_HOST + TAIGA_USERNAME/TAIGA_PASSWORD, with TAIGA_TLS_VERIFY support.
  • cli.py — standalone entry point with equivalent CLI flags for running the server directly.
  • serialize.py — helper to make Taiga model objects JSON-serializable for tool responses.
• AGENTS.md — instructions for registering the Taiga MCP server with an agent, clarifying token vs. password auth.
• docs/mcp.rst — new Sphinx page documenting installation (the mcp extra), configuration, running standalone, registering with an MCP client, the full tool list, and a security note on the blast radius of write tools. Wired into docs/index.rst.
• tests/test_mcp_server.py — coverage for the new server.
• Packaging: setup.cfg gains an mcp extra (fastmcp dependency); MANIFEST.in fixed to include AGENTS.md and correct a requirements-tests.txt → requirements-test.txt typo (was failing check-manifest in the pypi-description tox env).
• tox.ini: restored the testenv:docs section (previously dropped while docs stayed in envlist) so all tox environments run cleanly.
• changes/267.feature towncrier fragment added.

References

Fix #267

Checklist

• I have read the contribution guide
• Code lint checked via inv lint
• changes file included
• Usage documentation added (docs/mcp.rst, AGENTS.md)
• Tests added

[codecov]

Codecov Report

❌ Patch coverage is 97.51037% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.68%. Comparing base (791b857) to head (c314fee).

Files with missing lines	Patch %	Lines
taiga/mcp_server/serialize.py	64.70%	3 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #268      +/-   ##
==========================================
+ Coverage   96.47%   96.68%   +0.21%     
==========================================
  Files           8       12       +4     
  Lines         936     1177     +241     
  Branches       67       85      +18     
==========================================
+ Hits          903     1138     +235     
- Misses         19       22       +3     
- Partials       14       17       +3     

Flag	Coverage Δ
unittests	96.68% <97.51%> (+0.21%)	⬆️

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[coveralls]

coverage: 98.131% (+0.2%) from 97.97% — feature/issue-267-add-mcp into master