chat: retire the old saved-session store (Stage 4a)

Deletes all session persistence that isn't Claude's native transcript store, per
the "only harness-native session management survives" rule.

Backend: delete api/pi/storage.ts (meta.json+messages.json file store), the
api/saved-sessions router (+ unmount), the /pi/sessions REST endpoints, and the
storage.save/loadSession calls in the chat WS handler (in-memory session-manager
stays for live turns; no disk persistence — Claude's transcript is the record).
Also drops the Postgres saved_sessions layer: schema/chat.ts, queries/saved-sessions.ts,
its types and re-exports.

Frontend: delete state/useSavedSessions, ChatList, and the ChatHistory Widget
(all pure saved-session UI); slim ChatHeader to a label; strip the auto-load-latest
+ Save wiring from ChatPanelWrapper and ChatDetailPanel; drop the old resume path
from useChat and SessionListPage; remove the /chat/saved/:id route and the
useInitialData prefetch.

Behavior removed (intended): the Save-session button, email/project panels
auto-resuming the last chat, and /chat/saved/:id. /chat itself is unchanged —
already fully on Claude transcripts. The orphaned saved_sessions Postgres table
is dropped on the next `bun db:push`.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/apps/officer-web/state/useInitialData.ts

@@ -1,4 +1,3 @@
-import { useSavedSessions } from 'state/useSavedSessions';
 import { usePlans } from 'state/usePlans';
 import { useSettings } from 'state/useSettings';
 import { usePiModels } from 'state/useModels';
@@ -6,12 +5,11 @@ import { useAccessPolicy } from 'state/useAccessPolicy';
 import { useColorModeSync } from './useThemeSync';
 
 export const useInitialData = () => {
-  const { sessions } = useSavedSessions();
   const { plans } = usePlans();
   const { settings } = useSettings();
   usePiModels();
   useAccessPolicy();
   useColorModeSync();
 
-  return { sessions, plans, settings };
+  return { plans, settings };
 };