The landing pass holds a pull that is red alone with `forRed`, and the stale red-alone holds are re-placed or released #6758

Replaces #6753, which two builders declined because forRed (from #6348, pull #6357) was not on main yet. This waits for #6348, so it starts only when forRed exists.

Done when:

• [ ] Wherever the steward or landing pass holds a pull because its check is red alone, it places the hold with forRed: true rather than a person-style hold (find the call with git grep -n "red alone" -- src).
• [ ] A one-time pass on boot (or a migration) finds automatic holds whose reason is "red alone" and that predate forRed: it re-places each as a forRed hold, or releases it with a one-line note when its head is already green. A hold a person placed is never touched.
• [ ] The PUT hold API, studio and tty keep placing person holds; nothing there changes.
• [ ] Tests: the landing pass's red-alone hold carries forRed; the boot pass converts an old automatic red hold, releases one on a green head, and leaves a person's hold alone. Pass now in, never the wall clock.

Dropped, with #6847: the landing pass marks a pull "red alone" and hands it back; it places no pull hold, and forRed will not exist. A person's hold placed until: "green" already releases on green (#6508). The stale holds that never clear are #6425's.