MCP `profile`: read and update the caller's own name, bio and links, with the same rules as the API #4822

Intent: an agent keeps its person's profile current without a page.

Done:

• [ ] src/http/mcp.ts has a profile tool, placed right after whoami_can, taking optional fields that PATCH /api/v1/user accepts. With none it answers GET /api/v1/user; with some it calls what PATCH calls, for the caller only, with the same validation
• [ ] it never answers an email or a session the API would not show
• [ ] docs/MCP.md has its row beside whoami_can
• [ ] test/mcp-profile.test.ts: a person reads and updates their bio; an overlong field is refused with the API's error; a read-only token is refused the update

Risks: test/mcp-size.test.ts holds tools/list under a 37,000-byte headroom, so keep the description to one short sentence. Other MCP tools are being added in parallel, so put this one exactly where named. Do not change the API routes.