fix(ci): remove Project image digest migration gate

[mzxchandra]

Summary

The Project-specific image-digest gate added by #8590 blocks every later staging deployment while the Project migration is pending, including unrelated PRs. Remove that gate and its manually populated GitHub variables from the shared migration workflow.

Delete the unused Python checker and its tests, and remove the migration job's AWS secret forwarding and OIDC permission. Retain the database prerequisite check, migration ordering, authority switch, transactional backfill, constraints, and completion-receipt validation unchanged. No replacement deployment-verification infrastructure is added.

Rollout

Keep this PR in draft until the one-time staging compatibility and relevant worker-drain check is complete. Merging it allows the pending migration to proceed through normal CI; removing the gate is not evidence that the rollout is safe. Production requires its own compatibility verification before this change is deployed there. Supported rollback remains #8830 or compatible newer code.

Testing

• Local: actionlint 1.7.12 passed for both changed workflows; git diff --check passed.
• Verified there are no remaining references to the removed checker or readiness variables in workflows/scripts.
• Database code is unchanged; full validation runs in hosted PR CI.
• No migration, deployment, or workflow rerun was performed for this PR.

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Project	Deployment	Actions	Updated
docs	Building	Preview	Oct 13, 2026 3:49am UTC

[mzxchandra]

@greptile

[mzxchandra]

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[cubic-dev-ai]

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[cubic-dev-ai]

No issues found across 4 files

Confidence score: 5/5

• Automated review surfaced no issues in the provided summaries.
• No files require special attention.

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[greptile-apps]

Confidence Score: 5/5

[Critical impact] No new blocking code issue was found; merge remains subject to the staging checks stated in the rollout plan.

Summary

Removes the Project image-digest gate from the shared migration workflow.

• Project migrations proceed without an image-digest deployment gate.

Diagram

%%{init: {'theme': 'neutral'}}%%
flowchart TD
  A[CI checks pass] --> B[Check database prerequisites]
  B -->|Pass| C[Run database migrations]
  B -->|Fail| D[Stop job]
  C --> E[Promote application image]

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