Avoid Modal manifest transitions for code-only deploys

[anth-volk]

Fixes #1529

Summary

• Treat PR-body modal_release YAML as the canonical signal for current/frontier release operations.
• Use code-only deploy mode when the finalized versioning commit has no Modal release YAML: redeploy active manifest workers and gateway without mutating the manifest.
• Remove source_commit from manifest references and add a manifest rewrite command for cleaning existing Modal manifests.
• Preserve each active manifest app's US/UK package versions during code-only worker redeploys.
• Make retired app cleanup tolerate Modal's already-stopped response.

Testing

• uv run --extra dev pytest -vv --timeout=150 -rP .github/scripts/test_resolve_modal_release_config.py .github/scripts/test_modal_deploy_release.py .github/scripts/test_modal_cleanup_apps.py tests/unit/modal_release/test_manifest.py tests/unit/modal_release/test_worker_app.py .github/scripts/test_check_modal_release_body.py (passed before final formatting)
• ruff check .github/scripts/resolve_modal_release_config.py .github/scripts/modal_active_worker_apps.py .github/scripts/test_resolve_modal_release_config.py .github/scripts/test_modal_deploy_release.py .github/scripts/test_modal_cleanup_apps.py policyengine_household_api/modal_release/manifest.py policyengine_household_api/modal_release/update_manifest.py policyengine_household_api/modal_release/rewrite_manifest.py policyengine_household_api/modal_release/images.py tests/unit/modal_release/test_manifest.py tests/unit/modal_release/test_worker_app.py
• bash -n .github/scripts/modal-deploy-release.sh .github/scripts/modal-cleanup-apps.sh
• make format-check

Note: make format-check printed the local Docker daemon warning while Make expanded Docker helper variables, but ruff format --check . completed successfully. Per request, I did not rerun pytest after formatting.