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/workspaces/apps/Chat/QuestionActivity.tsx

@@ -1,8 +1,8 @@
 import { useState } from 'react';
 import { MessageCircleQuestion, Check } from 'lucide-react';
-import type { LegacyChatMessage } from './types';
+import type { ChatMessage } from './types';
 
-type ToolMessage = Extract<LegacyChatMessage, { role: 'tool' }>;
+type ToolMessage = Extract<ChatMessage, { role: 'tool' }>;
 
 type QuestionOption = {
   label: string;