Reflection follow-up: rewrite missing substrate file pr_safe_close_test.sh

Missing path: ~/shared-knowledge/scripts/test/pr_safe_close_test.sh Referenced in: PR-DEDUP-PROTOCOL.md line 180. Presumed contents: Hermetic test suite for pr-safe-close.sh. PR-DEDUP-PROTOCOL.md says it runs in CI. Likely a bash test script that mocks gh API responses and verifies the four preconditions are enforced. Should be created after pr-safe-close.sh is restored (task for pr-safe-close.sh should be done first).