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>

docs/INVENTORY-MANIFEST.json

@@ -274,6 +274,7 @@
       "active-workstream-store.cjs",
       "adr-parser.cjs",
       "agent-command-router.cjs",
+      "agent-install-check.cjs",
       "artifacts.cjs",
       "audit-command-router.cjs",
       "audit.cjs",