Fail loud on a dropped #oss-alerts page: the failure-alert cross-post no longer
swallows a 200/ok:false Slack response, so a dropped page-the-humans alert reds
the renderer job instead of vanishing on a green run. The thread reply stays
warn-only. Both posts capture the response via a shared slack_alert_posted_ok
predicate, mirrored byte-identically across the live workflow and the dry-run
helper.
Debt cleanup: drop a dead failed_count var, correct a misleading gha_url comment,
and validate the decoded blob run_id against ^[0-9a-f]{6}$ in the render step so
a malformed run_id can't reach Slack or the run name.
Tests: predicate edge cases (non-JSON, malformed, missing/null ok), an anti-drift
parity guard asserting the predicate is identical in both files, and call-site
tests locking the #oss-alerts fail-loud vs thread warn-only exit semantics.
Jordan Ritter