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/state/useSlashCommands.ts

@@ -19,7 +19,9 @@ export const useSlashCommands = ({ sessionId }: UseSlashCommandsParams) => {
 
     switch (command) {
       case 'rename':
-        return renameSession('claude', sessionId, args);
+        if (!sessionId || !args) return { handled: false };
+        await renameSession(sessionId, args);
+        return { handled: true, feedback: `Session renamed to "${args}"` };
       default:
         return { handled: false };
     }