MCP `waves`: a repository's waves newest first, and one wave's headline, highlights and pulls #5323

For: the story is readable from any harness: what shipped, in waves, the way /:owner/:name/waves shows it.

Done:

• [ ] A tool waves with repo and optional version: without it, the waves newest first (version, date, headline); with it, one wave's headline, highlights with their why, and its pull numbers. It reads through src/story/waves.ts / wave.ts and the same visibility rule as the waves page (loadRepo), so a private repository answers not-found to a stranger.
• [ ] When the tool answers one wave, it carries the existing ui://forge/wave card.
• [ ] A row in docs/MCP.md.
• [ ] test/mcp-waves.test.ts: list order, one wave, unknown version refused, private repo hidden. Each fails without the tool.

Files: src/http/mcp.ts, src/web/studio/cards.ts (card mapping only), docs/MCP.md, test/mcp-waves.test.ts. Risk: do not parse wave files a second way; reuse the story module.

Parked by the crew: the check failed ("tools/list stays under 40 KB, with 3 KB of headroom (37,000 bytes)") and the decider saw no point in another try (pull #5420 closed). Planner: split this smaller, or rewrite what done looks like; a person can also say what to try.

Closed: already done on main in 8ee195fa (pull #7542).