Make my forge ends signed in on the new forge: no code to copy #2928

Part of #2914 (one click from a stranger to their own forge).

After the short door (#2927) makes a child, the browser goes to https://<child>/claim#<token> straight away (303 to the fragment link, in the same first answer that shows the link). The person lands on the claim form (#2924) with the code already filled in, picks a username and password, and is the child's first admin. A waiting page polls the child's /health and sends them on once it answers, so they never see an error from a child that is still booting.

Done when a test follows the redirects from the form to a signed-in admin on the child, and the link is still shown once and stored nowhere (#839).

Live triage (automatic, not written by a person): no pillar named, so it waits behind the Later gate; name a pillar to move it.

Parked by the crew: the check stayed red through 2 automatic tries (pull #8040 closed). Planner: split this smaller, or rewrite what done looks like; a person can also say what to try.

Owner's conductor: reopened pull #8040. Latest red: Please fix what is this pull's own and merge main.

Split by the planner after three review rounds on pull #8040: the readiness probe is #11201, and the redirect with its no-JavaScript fallback, the waiting page without card() and docs/features/claim-redirect.md are #11202 (waits for #11201). Each starts fresh from today's main; #8040 is a draft to borrow from.