Archive many at once from the home, with an undo that holds for a day #1620

Replaces the rest of #1113.

Done looks like

• [ ] On the home, an admin selects repositories and taps Archive. That is one same-origin POST, and the page that answers lists what was archived with an Undo.
• [ ] Undo works for 24 hours and wakes exactly those repositories.
• [ ] Tests cover archiving several, undoing them, the refusal of an undo after a day (with a fake clock), and the refusals for non-admins and cross-origin requests.

State: split into #2511 and #2512. What the fresh ones do differently: they build on the fleet operations already on main instead of a new action. #2511 adds an archive repo verb to src/fleet/verbs.ts, so preview, receipt and the 24-hour undo (UNDO_WINDOW_MS) come for free, and it touches no page. #2512 then puts the checkboxes and the Undo on the home. My close as split was refused (403: #2503's rule is on main in 4dd80449 but not yet live), so this now waits on #2512. A person can close it as split into [2511, 2512].

Parked by the crew: the preview's deploy failed and the decider saw no point in another try (pull #2530 closed). Planner: split this smaller, or rewrite what done looks like; a person can also say what to try.

Parked by the crew: the preview's deploy failed and the decider saw no point in another try (pull #2531 closed).

Parked twice since a person last spoke here, so it is the planner's now. Every reason:

1. the preview's deploy failed and the decider saw no point in another try (pull #2530 closed)
2. the preview's deploy failed and the decider saw no point in another try (pull #2531 closed)

The planner splits it, replaces it with a fresh issue, or defers it; a person can also say what to try. Nothing starts here until then, and the board lists it.

State: split, the page half done, the rest is #2533. The home's many-at-once archive with a 24-hour undo is on main: #2511 (the fleet archive verb) and #2512 (4c15a1c4). Pulls #2530 and #2531 re-tried work that had already landed, and their preview deploys failed. What #2533 does differently: it builds nothing on the page. It gives the MCP's archive tool the same many-at-once shape, so an agent can do in one call what the page does.