refactor(#1289): T5 — migrate the final idiom-hard callers off the core spine (#1290)

The last 4 core importers, migrated off non-destructure idioms:
- gsd-tools.cjs: core.{error,ERROR_REASON,setJsonErrorMode,output} -> io.cjs;
  core.findProjectRoot -> project-root.cjs (lazy wrapper preserved); inline
  resolveWorktreeRoot require -> worktree-safety.cjs
- audit-command-router: DI default `_core ?? core` -> `_core ?? io` (seam preserved)
- intel-command-router: DI default -> `{ output: io.output, timeAgo: coreUtils.timeAgo }` (seam preserved)
- check-command-router: io destructure -> io.cjs; dynamic core['planningDir']
  -> planning-workspace, core['findPhaseInternal'] -> phase-locator (typed
  imports, dropped the Record-cast bracket hack)

Allowlist is now EMPTY — NO file imports the core spine. core.cts re-exports
are dead weight; T-final deletes core.cts + remaining shim tests + the lint.
No behaviour change.

Closes #1289

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

scripts/lint-core-spine-imports.allowlist.json

@@ -1,9 +1,4 @@
 {
   "_comment": "Files allowed to import the core re-export spine during the staged retirement (issue #1268). Entries are REMOVED as each tranche migrates a leaf; the file + this lint are deleted in T-final.",
-  "allow": [
-    "gsd-core/bin/gsd-tools.cjs",
-    "src/audit-command-router.cts",
-    "src/check-command-router.cts",
-    "src/intel-command-router.cts"
-  ]
+  "allow": []
 }