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

PI_HARNESS_REBUILD.md

@@ -1,6 +1,6 @@
 # Pi Harness Rebuild — Implementation Plan
 
-**Status**: 🚧 IN PROGRESS — Phase 7 (Frontend Migration)  
+**Status**: ✅ COMPLETE — Phase 9 (Final Cleanup)  
 **Date**: February 20, 2026  
 **Scope**: Replace all three legacy harnesses (Claude, OpenCode, Pi-Mono) with single, clean Pi harness
 
@@ -17,21 +17,21 @@
 - [x] **Phase 6**: Cleanup & Polish — Complete
 - [x] **Phase 6.1**: Session Grouping — Complete
 
-### Frontend (In Progress)
+### Frontend (Complete)
 - [x] **Phase 7.1**: Type Alignment — Complete ✅
-- [ ] **Phase 7.2**: Unified Pi Hook (`usePi.ts`) ⭐
-- [ ] **Phase 7.3**: Unified Models Hook
-- [ ] **Phase 7.4**: Session Management Migration
-- [ ] **Phase 7.5**: Group Support Hooks
+- [x] **Phase 7.2**: Unified Pi Hook (`usePi.ts`) — Complete ✅
+- [x] **Phase 7.3**: Unified Models Hook — Complete ✅
+- [x] **Phase 7.4**: Session Management Migration — Complete ✅
+- [x] **Phase 7.5**: Group Support Hooks — Complete ✅
 
-### UI Enhancements (Planned)
+### UI Enhancements (Future)
 - [ ] **Phase 8.1**: Grouped ChatList UI
 - [ ] **Phase 8.2**: Group Management UI
 - [ ] **Phase 8.3**: Search Enhancements
 
-### Final Cleanup (Planned)
-- [ ] **Phase 9.1**: Frontend Legacy Cleanup
-- [ ] **Phase 9.2**: Backend Final Cleanup
+### Final Cleanup (Complete)
+- [x] **Phase 9.1**: Frontend Legacy Cleanup — Complete ✅
+- [x] **Phase 9.2**: Backend Final Cleanup — Complete ✅
 
 ---