refactor(#1268): re-home core re-export-spine squatters + migration-convergence lint (#1272)

Re-home the 6 implementation functions squatting in the core.cjs re-export
spine (ADR-857) into the modules whose interface they belong to, with core
re-exporting them BY REFERENCE so all 32 callers + the shim-identity tests
keep resolving unchanged:
- worktree-safety: resolveWorktreeRoot, pruneOrphanedWorktrees
- git-base-branch (broadened to the Git Query Module): gitWorktreeInfoInternal
- agent-install-check (new leaf): getAgentsDir, checkAgentsInstalled
- delete the _resetRuntimeWarningCacheForTests wrapper; consumers use a
  shared resetRuntimeWarningCaches() helper in tests/helpers.cjs

Add scripts/lint-core-spine-imports.cjs (migration-convergence lint with a
30-importer allowlist, wired into lint:ci) so the staged spine retirement
provably converges: CI fails on any new ./core import. Register the new
generated agent-install-check.cjs in eslint-ignore + .gitignore +
INVENTORY-MANIFEST.json.

No behaviour change. First tranche (T0) of epic #1267.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

package.json

@@ -92,7 +92,7 @@
     "pretest:coverage": "npm run build:lib && npm run lint:skill-deps",
     "lint": "eslint . --cache --cache-location node_modules/.cache/eslint/",
     "lint:fix": "eslint . --fix",
-    "lint:ci": "npm run lint && npm run lint:skill-deps && 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-windows-test-portability.cjs && node scripts/lint-allow-test-rule-refs.cjs",
+    "lint:ci": "npm run lint && npm run lint:skill-deps && 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-windows-test-portability.cjs && node scripts/lint-allow-test-rule-refs.cjs && node scripts/lint-core-spine-imports.cjs",
     "lint:allow-test-rule-refs": "node scripts/lint-allow-test-rule-refs.cjs",
     "lint:windows-test-portability": "node scripts/lint-windows-test-portability.cjs",
     "lint:regression-names": "node scripts/lint-regression-test-names.cjs",