Roll out queued auto-merge evaluations and retire the temporary opt-in

[devantler]

🤖 Generated by the Agentic Engineer

The opt-in queue-pending-evaluations flag introduced by #1151 stages the #725 repair because retaining pending jobs can increase runner/API work. Existing callers and direct/required runs keep their current queue until rollout.

Review date / target expiry: 2026-09-12. This is a temporary release flag, not a permanent setting. The expiry is a review deadline, never permission to remove it without evidence.

Acceptance:

• After the delivery merges, enable the flag on a small set of trusted workflow_call consumers and observe a real event burst plus steady-state operation.
• Record retained/completed/cancelled evaluations, execution time, and API rate-limit errors; verify no stale authorization or revocation regression. Do not publish financial data or mutate PRs solely to generate evidence.
• Define and validate direct/required-workflow activation, preserving explicit workflow_call false while the flag exists.
• Progressively enable the default after successful validation, then remove the input and temporary activation plumbing; keep cancellation guards and queue tests.
• Close Auto-merge gate can silently skip an evaluation: adopt concurrency.queue: max #725 only once its production acceptance is demonstrated. If rollout is deferred at the review date, record a specific blocker and a new bounded date here.

GitHub's queue limit remains 100 pending jobs/runs; ordering follows the start of waiting rather than event dispatch. No App authority expansion is part of this task.