feat: gateway CRUD routes for conversations, projects, missions, tasks (P1-005, P1-006)

- BrainModule provides Brain instance to all gateway controllers
- ConversationsController: CRUD + message listing/creation
- ProjectsController: CRUD with owner assignment
- MissionsController: CRUD with project association
- TasksController: CRUD with project/mission/status filtering
- All routes guarded by AuthGuard (BetterAuth session validation)
- DTOs at module boundaries per project convention

Closes #14, Closes #15

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

apps/gateway/package.json

@@ -14,6 +14,7 @@
   "dependencies": {
     "@mariozechner/pi-coding-agent": "~0.57.1",
     "@mosaic/auth": "workspace:^",
+    "@mosaic/brain": "workspace:^",
     "@mosaic/db": "workspace:^",
     "@nestjs/common": "^11.0.0",
     "@nestjs/core": "^11.0.0",