ci: fail the review job when the reviewer errors

[andybons]

Summary

Closes the false-green class observed repeatedly: when the review action's model call fails (API error / usage-window cap), it writes a placeholder comment and records is_error in its execution output, yet the job exits success — producing a mergeable-looking PR with zero findings that was never reviewed. A verification step now inspects the execution-output JSON and fails the job (red check) when the review errored, so an exhausted usage window shows up as a visible failure instead of a silent pass. Dependency-free (jq, preinstalled).

Goal-supervised box build (ses_2621420c5a72d03b), box-pushed.

🤖 Generated with Claude Code

[andybons]

Merging with the claude-review check red — that red is this PR's own verify step working as designed: the review action skips itself on PRs that modify the workflow file (supply-chain protection; it requires the file to match the default branch), so workflow PRs can never self-review. Coordinator review of the 28-line diff performed directly: fail-closed on missing output, defensive object/array handling, if: always(), correct is_error check. Test job green. The action's own log says the workflow activates on merge.