Complete frontend migration to unified Pi harness (Phase 7 + Phase 9)

- Delete legacy hooks: useClaude.ts, useOpenCode.ts, usePiMono.ts, App.backup.tsx
- Update all components to use usePi instead of legacy hooks
- Replace useVisiblePiMonoModels/useClaudeModels/useOpenCodeModels with useVisiblePiModels
- Migrate from LegacyChatMessage to ChatMessage type throughout
- Update SessionBar to remove provider and archive props
- Simplify ChatDetailPanel to Pi-only (remove Claude/OpenCode components)
- Fix useChatSessions calls (remove provider parameter)
- Update user-settings types: provider now only 'pi' instead of legacy values
- Update PI_HARNESS_REBUILD.md to mark phases complete

src/apps/officer-web/Screens/Dashboard/ChatHistory/index.tsx

@@ -32,12 +32,12 @@ export const SessionListPage = ({ isNew }: SessionListPageProps) => {
 
   useEffect(() => {
     if (isNew) {
-      setSelected({ id: 'new', provider: 'pi-mono' });
+      setSelected({ id: 'new' });
       return;
     }
     if (!sessionId) return;
     const session = sessions.find((s) => s.id === sessionId);
-    setSelected({ id: sessionId, provider: session?.provider ?? 'pi-mono', model: session?.model ?? null });
+    setSelected({ id: sessionId, model: session?.model ?? null });
   }, [sessionId, isNew]);
 
   const panelComponents: PanelComponents = useMemo(