feat(ci): add forgotten-sibling-tests quality gate (#9530)

New CI policy gate that detects when a PR changes source file Y but the test siblings of Y's consumers are not in the same diff. Prevents the 'forgotten sibling test' pattern (7 documented occurrences in PR #9529).

- Extracts shared import resolution library (resolveImport, sourceDepsOf) from build-test-impact-map.mjs
- Creates check-forgotten-sibling-tests.mjs gate with consumer-walk logic
- Adds forgotten-sibling-allowlist.json for false positive mitigation
- Wires into pr-test-policy CI job and adds npm script
- Documents in QUALITY_GATES.md
- 17 TDD unit tests for isSourceFile, resolveBase, testSiblingOf, isAllowlisted, findConsumers

.github/workflows/ci.yml

@@ -587,6 +587,9 @@ jobs:
       # Anti test-masking: flag net assert removal / new assert.ok(true) in changed tests.
       - name: Detect test-masking (weakened assertions)
         run: npm run check:test-masking
+      # Detect forgotten sibling tests (consumers whose test sibling is not in the diff).
+      - name: Detect forgotten sibling tests
+        run: npm run check:forgotten-sibling-tests
       # Evidence-in-PR-body (Hard Rule #18 mechanized): claims of "tests pass" must carry output.
       - name: Require evidence in PR body
         run: npm run check:pr-evidence