chore(lifecycle): remove quarantined state (phantom)

State existed in the lifecycle table and the enum but no verb, route,
or service path ever set it. Removing dead state. If we need
problem-task isolation later we'll add it explicitly with a verb.

docs/rag/workflows/task-states.md

@@ -20,9 +20,6 @@
 - `completed` - Work finished
 - `cancelled` - Work cancelled
 
-### Special States
-- `quarantined` - Problematic task, can return to pending
-
 ### Setup State
 - `backlog` - PM setup phase, not ready for work