Add a local MCP server for the documentation corpus

[miharp]

Summary

Adds a local Model Context Protocol server
(tools/mcp/) that exposes the OpenVox documentation to MCP-aware tools (Claude
Code, Cursor, GitHub Copilot, Codex, …) so an assistant can search and read the
docs in-workflow. It is the local, self-hosted counterpart to a hosted "Ask AI"
widget: no third party, no API key, no quota, and queries stay on the machine.

Implements #364. Depends on #358 for the live-corpus default (see below).

How it works

The corpus is the site's llms.txt / llms-full.txt files, which already ship
as machine-readable plain text with stable per-project (# Project:) and per-doc
(--- / ## title / Source:) delimiters, so the server parses them into one
document per page without scraping HTML.

• corpus.py — fetches the two files from the live site by default, with
  on-disk caching + conditional requests and an offline fallback to the last good
  copy; OPENVOX_DOCS_SOURCE reads a local _site/ build instead.
• search.py — BM25 keyword ranking (rank-bm25) over the parsed bodies,
  title-weighted, with a query-centered snippet.
• server.py — FastMCP server exposing the tools below.

Tools

Tool	Purpose
list_projects	List the documentation projects.
list_docs(project?)	List pages as {project, title, url}.
search_docs(query, project?, limit)	BM25 search → {title, url, project, score, snippet}.
get_doc(ref, max_chars=40000)	Full text of one page (URL/path/title); capped, since the single-page references are large.
refresh_corpus()	Force a re-fetch and rebuild the index.

Tests & CI

• pytest — 30 unit tests covering corpus parsing, remote fetch/caching (304
  reuse + offline fallback), BM25 ranking, and every tool (~93% coverage).
• smoke_test.py — launches the server over stdio against a throwaway corpus and
  exercises all five tools end-to-end (network- and model-free).
• .github/workflows/mcp.yml — runs both on Python 3.10 and 3.13, path-filtered
  to tools/mcp/**.

Dependency on #358

The default corpus source is the live site, which 404s until #358's llms.txt /
llms-full.txt are deployed. Until then the server runs against a local build via
OPENVOX_DOCS_SOURCE=$PWD/_site (documented in the README). Opening as a draft
for that reason; ready to finalize once #358 ships.

Notes

• New subproject with its own Python toolchain; excluded from the Jekyll build.
• The demo GIF is committed and embedded in tools/mcp/README.md, regenerable
  from tools/mcp/demo/demo.tape with VHS.