[Reliability] Propagate scheduler mutation failures to a non-passing workflow result

[seonghobae]

Problem

scripts/ci/pr_review_merge_scheduler.py::main() deliberately isolates PR-local mutation failures: it converts a caught RuntimeError into an action_error decision and continues inspecting the rest of the queue. That isolation is useful, but the CLI then unconditionally returns 0.

As a result, an update, merge, auto-merge, or dispatch mutation can fail, appear only in the structured summary as action_error, and still leave the scheduler job/workflow with a passing process result. The current tests explicitly assert this behavior.

Impact

A green scheduler check can be mistaken for successful mechanical governance even though a requested mutation failed. This weakens operational alerting and makes consumers parse logs or JSON to discover a material action failure.

Acceptance criteria

• Preserve PR isolation: one failed mutation must not prevent later PRs from being inspected.
• After the full bounded scan, emit the existing structured per-PR summary.
• Return a non-zero terminal result when one or more material mutation/dispatch attempts ended in action_error.
• Keep ordinary policy states such as wait, blocked, skip, or deferred capacity distinguishable from execution failure.
• Ensure the reusable/required workflow exposes the failure without losing the summary artifact/job summary.
• Add regression coverage for:
  • one action_error followed by an unaffected PR;
  • multiple action_error decisions;
  • no-action-error success;
  • structured output retained before terminal failure;
  • workflow consumer propagation.
• Document whether a targeted single-PR run and an organization sweep use the same terminal policy.

Evidence

• scripts/ci/pr_review_merge_scheduler.py catches RuntimeError per PR and creates Decision(..., "action_error", ...), then returns 0.
• tests/test_pr_review_merge_scheduler.py::test_main_keeps_scanning_after_action_error and the update-branch 403/422 test assert that the CLI returns 0 while the summary contains action_error.

Related work

• [Governance] Align scheduler merge modes and mutation credential authority #892 owns merge-mode and mutation-credential authority alignment.
• [Reliability] Add a cross-workflow branch writer lease with fencing #890 owns the cross-workflow writer lease and fencing contract.

This issue is about terminal workflow truthfulness, not credential selection or queue serialization.