Fix verdict parsing + plug stale-task enforcement hole

- task.py: remove naive substring fallback from parse_verdict_status(),
  only parse ## Status: header; no header → ambiguous (REFEREE)
- status.py: add _auto_update_verdict_on_complete() — when transitioning
  human_intervention→complete, auto-update VERDICT.md to PASS
- status.py: add stale-task detection to --can-edit — deny edits if
  all edit tasks have .state mtime >30 min old (reason: stale_task)
- status.py: add --touch command to reset task activity clock
- guard plugin: handle stale_task reason with specific error message
- Update test to match new verdict parsing behavior

tasks/plug-stale-task-hole/SPEC.md

@@ -0,0 +1 @@
+# Plug Stale Task Hole\n\nAdd stale-task detection, --touch command, update guard plugin.