MCP `preview`: for a preview URL, the app, the pull it previews, its check state and its log link, as the forge pill shows

The same action has the same shape for a person and an agent. An agent given a preview link should learn what the pill tells a person.

Done when:

[ ] MCP preview takes { url } and answers { app, environment, pull: { number, title } | null, check: "green"|"red"|"running"|null, firstFailure, logUrl, working: [names] }.
[ ] It refuses a URL that is not one of this forge's app hosts, and a pull the caller cannot see, with the same words the API uses.
[ ] It is described in docs/MCP.md and listed in tools/list.
[ ] Tests cover a production URL, a preview URL with a red check, a foreign host and an unseen private pull.

MCP `preview`: for a preview URL, the app, the pull it previews, its check state and its log link, as the forge pill shows #5828