MCP `app_domains`: list, add, check and remove an app's domains, with the same rules as the API #4753

Intent: an agent wires a custom domain for an app and reads its DNS state through MCP.

Done:

• [ ] src/http/mcp.ts has an app_domains tool, placed right after app_members, taking repo, action (list | add | check | remove) and host, and calling what /api/v1/repos/:owner/:name/app/domains and …/domains/:host[/check] call: the same validation of the host, the same refusals
• [ ] the answer carries the DNS records the page shows
• [ ] docs/MCP.md has its row beside app_members
• [ ] test/mcp-app-domains.test.ts: an admin adds, checks, lists and removes a domain; a non-admin is refused; an invalid host is refused

Risks: test/mcp-size.test.ts holds tools/list under a 37,000-byte headroom; keep the description to one short sentence, and if the headroom fails after a rebase, shorten this tool's own description, not another's. Other MCP tools are being added in parallel: put this one exactly where named so the hunks do not meet.