An app host's /__forge/ door shows the exact ssh command for the app's terminal to its writers, or says the terminal is off #5957

From any app you should know how to get into it. sshCommand(app, login) (src/ssh/command.ts) already builds the exact command for an app, and for slug--N for a preview.

Done when:

• [ ] For a viewer who may write the app's repository, the door (src/apps/door.ts) shows that command verbatim in a copyable code line. It uses the preview's login on a preview and the app's own login on production. It never invents a host or user.
• [ ] When sshCommand answers undefined, or the app declares no tty, the door says The terminal is off.
• [ ] Readers who cannot write, and signed-out visitors, see neither.
• [ ] Tests: a writer on production and on a preview, a reader, and SSH off. Do not touch src/http/mcp.ts, src/contract/openapi.ts or docs/DESIGN.md.