BLD-001 K3: gate admission as code (first-green receipt, p90 ≤50% of limit, hours budget) + projected-finish alarm #4792

Problem. Required checks were admitted with no proof that they can go green. The mutants shard p90 is 720 min, which is its timeout, and 0 of 11 runs were green. Nothing alarms when a check's projected finish passes its limit.

Change (net-new only).

1. Gate admission as code: a check may become required only with all three of these:
   • a first-green receipt;
   • p90 at most 50% of its timeout;
   • an hours budget.
2. A projected-finish alarm that fires within 15 min of a run's projection crossing its limit.

Already filed and not part of this ticket: #4649, #4653, #4648 (restore), #4652, #4654.

Metric. 0 required checks without an admission receipt; alarm latency at most 15 min.

Falsifier. A required check with no first-green receipt is refused (RED); one with a receipt is admitted (GREEN).

Approved by one quorum round on 2026-10-05 (degraded: same-family; lanes claude-sonnet-5-5 ×2 + claude-haiku-4-5, author claude-opus-5-5). Parent: BLD-001 epic.