[Molecule-Platform-Evolvement-Manager]
## What was broken
Same bug class as the secret-scan.yml fix in #2120 — block-internal-paths
hit `fatal: bad object <sha>` exit 128 on the staging push at
2026-04-27 06:50:33Z.
Two cases:
1. **`merge_group` events**: BASE/HEAD came from
`github.event.before` / `.after` which are push-event-only
properties. On merge_group both came back empty, the script fell
through to "scan entire tree" mode which is correct but
inefficient. Worse, when this workflow is required for the merge
queue (line 21-22), an empty-BASE entire-tree scan would run on
every queue check.
2. **`push` events with shallow clones**: `fetch-depth: 2` doesn't
always cover BASE across true merge commits. When BASE is in the
payload but absent from the local object DB, `git diff` errors out
with `fatal: bad object <sha>` and the job exits 128. This is what
broke today's staging push.
## Fix
Same shape as the secret-scan.yml fix (#2120):
- Add a dedicated `git fetch` step for `merge_group.base_sha`.
- Move event-specific SHAs into a step `env:` block; script uses a
`case` over `${{ github.event_name }}` covering pull_request /
merge_group / push (rather than `if pull_request / else push`
which left merge_group on the empty-BASE branch).
- On-demand fetch + `git cat-file -e` guard for push BASE so a SHA
that's payload-present-but-DB-absent triggers the fetch, and a
fetch failure falls through cleanly to "scan entire tree" instead
of exiting 128.
## Test plan
- [x] YAML structure preserved (no schema changes)
- [x] Bash logic mirrors the secret-scan recovery path tested in #2120
- [ ] CI green on this PR's pull_request scan + push to staging post-merge
🤖 Generated with [Claude Code](https://claude.com/claude-code)
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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