Adds three canonical fixtures under showcase/test-fixtures/promote-notify/:
success.json (29 services green), partial.json (26 green / 3 red with
mixed exit codes + categories), and total-failure.json (29 red fleet
abort with truncation-suffix sentinel).
Includes validate.sh — strict schema enforcement (rc propagation, enum +
regex assertions for run_id, pre_staging, abort_reason, category, exit
codes). And a README documenting the fixture contract + how to dispatch
them via gh workflow run.
github-actions[bot]