Cut-over: move an app's domain from the platform to the forge with a preview check, a receipt and one-step undo #648

Pillar: P4 Forge in a link (#590), under Just go forge (#643). One run, fresh from main. Planned: held until the owner sends the crew (#642).

Outcome. When an app is at parity, the owner moves its domain to the forge app in one step, checked first and undoable.

Today. Apps can take domains (/api/v1/repos/:o/:n/app/domains); nothing moves one off the platform.

The path: the owner sees "at parity" on an app; primary action Cut over; next the receipt and the inventory.

Acceptance.

• [ ] Preview check: the forge app serves the domain's main paths with the same status codes as the platform (a list it shows).
• [ ] Cut over: the domain attaches to the forge app; the receipt says what changed (DNS or tunnel route, certificate) and the platform app is left running, unrouted.
• [ ] Undo in one step restores the old routing; the inventory (#M3) reads "cut over".
• [ ] Minimal, not bare: one primary next action; looks of the path at phone and desktop widths on the pull.

Done means reachable:

• Page: the app page, when at parity
• MCP: cut_over {repo, domain, apply?}
• Test: check passes or blocks; undo restores
• Look: the check and the receipt

Removes / says what it removes: nothing removed.

Start from: src/actions/domains.ts, src/apps/domains.ts, src/core/domains.ts.

Must not touch: the release workflow, the chart, the old platform's repositories (read only).

Live triage (automatic, not written by a person): filed under P4 Forge in a link, which it names.