fix: add regression-test-presence gate + missing tests for #2429/#2279 (#2563)

lint-fix-has-regression-test.cjs: new gate that fails if a fix(#NNNN)
or feat(#NNNN) commit has zero behavioral test files (*.test.cjs,
excluding auto-generated fixtures/baselines) in its diff. Wired into
lint:ci so it runs before PR creation.

Missing regression tests added:
- #2429: codex local scope does not set $HOME/.agents skills home;
  global scope does (tests/runtime-artifact-layout.test.cjs)
- #2279: map-codebase instructions say to overwrite existing dates,
  not just replace [YYYY-MM-DD] placeholders (tests/commands.test.cjs)

package.json

@@ -103,7 +103,7 @@
     "lint": "eslint . --cache --cache-location node_modules/.cache/eslint/",
     "lint:fix": "eslint . --fix",
     "lint:table-schema-drift": "node scripts/lint-table-schema-drift.cjs",
-    "lint:ci": "npm run lint && npm run lint:skill-deps && npm run lint:generated-sync && node scripts/lint-test-file-count.cjs && node scripts/lint-command-contract.cjs && node scripts/lint-pr-check-project-dir.cjs && npm run lint:legacy-name && node scripts/lint-regression-test-names.cjs && node scripts/lint-allow-test-rule-refs.cjs && node scripts/lint-resolution-provenance.cjs && node scripts/lint-portable-timeout.cjs && node scripts/validate-registry.cjs && node scripts/lint-table-schema-drift.cjs",
+    "lint:ci": "npm run lint && npm run lint:skill-deps && npm run lint:generated-sync && node scripts/lint-test-file-count.cjs && node scripts/lint-command-contract.cjs && node scripts/lint-pr-check-project-dir.cjs && npm run lint:legacy-name && node scripts/lint-regression-test-names.cjs && node scripts/lint-allow-test-rule-refs.cjs && node scripts/lint-resolution-provenance.cjs && node scripts/lint-portable-timeout.cjs && node scripts/validate-registry.cjs && node scripts/lint-table-schema-drift.cjs && node scripts/lint-fix-has-regression-test.cjs",
     "lint:allow-test-rule-refs": "node scripts/lint-allow-test-rule-refs.cjs",
     "lint:regression-names": "node scripts/lint-regression-test-names.cjs",
     "lint:descriptions": "node scripts/lint-descriptions.cjs",