feat: add AI assistant with in-session copilot and standalone chat with RAG

Implements three-phase AI assistant feature:
- Phase 0: RAG infrastructure with pgvector embeddings, Voyage AI integration,
  tree chunking service, and semantic search over team's flow library
- Phase 1: In-session copilot panel during flow navigation with contextual
  AI help, current step awareness, and suggested related flows
- Phase 2: Standalone AI chat page with persistent conversation history,
  pin/delete, and configurable retention policies (account-level)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/api/index.ts

@@ -18,3 +18,5 @@ export { maintenanceSchedulesApi, batchLaunchApi } from './maintenanceSchedules'
 export { default as feedbackApi } from './feedback'
 export { default as aiBuilderApi } from './aiBuilder'
 export { default as aiChatApi } from './aiChat'
+export { copilotApi } from './copilot'
+export { assistantChatApi } from './assistantChat'