A hold set "until green" releases itself when a green check lands on the pull's head, with a note that says so #6495

Part of #5930 (steward, 16:15Z item 2: "red alone, held" holds stayed on 7 pulls after they turned green).

Done:

• [ ] setHold in src/core/holds.ts takes an optional until: "green", stored in a new column by a migration (default: none, so today's holds are unchanged).
• [ ] When a check finishes green on a pull's current head and its hold is until: "green", the hold is released by the forge, and the pull's timeline carries one line: "hold released: green on <short oid>".
• [ ] A plain hold is never released by a green check.
• [ ] Tests: until-green hold released on green; plain hold stays; red check leaves both.

Files: src/core/holds.ts, a migration, the place a check verdict is recorded, a test.

Before pushing: bun x tsc --noEmit and the holds tests.