MCP `invites`: list the outstanding invites and revoke one, beside `mint_invite` #4793

Intent: an admin's agent that mints invites can also see which are still open and take one back.

Done:

• [ ] src/http/mcp.ts has an invites tool, placed right after mint_invite, taking action (list | revoke) and id. It calls what GET /api/v1/admin/invites and DELETE /api/v1/admin/invites/:id call (person admins only)
• [ ] list never shows an invite's link or token, only what the API's list shows
• [ ] docs/MCP.md has its row beside mint_invite
• [ ] test/mcp-invites.test.ts: an admin mints, lists and revokes; a revoked link no longer signs anyone up; a non-admin is refused

Risks: test/mcp-size.test.ts holds tools/list under a 37,000-byte headroom, so keep the description to one short sentence and, if that fails after a rebase, shorten this tool's own. Other MCP tools are being added in parallel, so put this one exactly where named so the hunks do not meet. Do not change the API route or what it allows: the tool calls the same functions.

Parked by the crew: the check failed ("a green agent pull and its reviewer > before the review it is the crew's: no ask, no count, a waiting word") and the decider saw no point in another try (pull #4885 closed). Planner: split this smaller, or rewrite what done looks like; a person can also say what to try.