Read an email campaign's shortlist at the revision admission pinned

[egeozin]

What and why

prepare_email_campaign found outreach/email/SHORTLIST.csv through canonical_file_history(limit=1), which scans only limit * 4 = 4 commits. Once four unrelated commits had landed, every campaign failed with "project state has no email shortlist revision", even though the shortlist was there.

Admission already pins where it found the shortlist: the campaign's required artifact prerequisite records {"path", "revision"} in run.prerequisite_evidence, refreshed after a settled wait. The campaign now reads the shortlist at that revision:

• Retries are stable. If an earlier attempt already created the outreach_campaigns row, the retry reuses its source_commit_sha, so create_email_campaign's idempotency check still matches, including for a run already in flight during a deploy.
• Older runs keep the old lookup. Runs admitted before artifact evidence existed fall back to the history scan.
• It reads what was admitted. Edits to the shortlist after admission don't change a campaign that's already running.

pinned_artifact_revision(evidence, path) in workflow_prerequisites.py reads a satisfied pin for one path and returns None otherwise.

Verification

• A new activity test in test_email_outreach.py covers both the admission pin and a recorded retry. Its fake storage refuses canonical_file_history, so the test fails on main and passes with the fix.
• A unit test for pinned_artifact_revision covers satisfied, unmet, other-path, malformed and legacy evidence.
• ruff format --check, ruff check and lint-imports are clean.
• Ran serially against a disposable local Postgres 14: test_email_outreach, test_email_campaign_temporal, test_workflow_prerequisites, test_prerequisite_wait, test_code_authors. All passed except test_prerequisite_wait::test_answer_page_holds_in_temporal_until_its_prerequisite_finishes. That test fails the same way on unmodified main in this local environment and doesn't touch this code.

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