feat(search): add PostgreSQL FTS on AI sessions with Command Palette integration

- Migration: add generated tsvector column + GIN index on ai_sessions (problem_summary, resolution_summary, escalation_reason, problem_domain)
- Backend: wire FTS into list_sessions q param; add GET /ai-sessions/search endpoint returning AISessionSearchResult (registered before /{session_id} to avoid UUID routing conflict)
- Frontend: add AISessionSearchResult type, aiSessionsApi.search() method, and Command Palette group "FlowPilot Sessions" using Zap icon navigating to /pilot/:id

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/api/aiSessions.ts

@@ -10,6 +10,7 @@ import type {
   SessionDocumentation,
   AISessionSummary,
   AISessionDetail,
+  AISessionSearchResult,
   PickupSessionRequest,
 } from '@/types/ai-session'
 
@@ -114,6 +115,13 @@ export const aiSessionsApi = {
     const response = await apiClient.get<AISessionSummary[]>('/ai-sessions/escalation-queue')
     return response.data
   },
+
+  async search(q: string, limit: number = 5): Promise<AISessionSearchResult[]> {
+    const response = await apiClient.get<AISessionSearchResult[]>('/ai-sessions/search', {
+      params: { q, limit },
+    })
+    return response.data
+  },
 }
 
 export default aiSessionsApi