feat(conversations): add full-text message search endpoint (M1-006)

Adds GET /api/conversations/search?q=<query> endpoint that searches
message content across all authenticated user's conversations using
PostgreSQL ILIKE. Supports limit/offset pagination (default limit=20)
and returns results sorted by recency with conversation context.

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

packages/brain/src/index.ts

@@ -25,6 +25,7 @@ export {
   type NewConversation,
   type Message,
   type NewMessage,
+  type MessageSearchResult,
 } from './conversations.js';
 export {
   createAgentsRepo,