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>

pnpm-lock.yaml

@@ -47,6 +47,9 @@ importers:
       '@mosaic/auth':
         specifier: workspace:^
         version: link:../../packages/auth
+      '@mosaic/brain':
+        specifier: workspace:^
+        version: link:../../packages/brain
       '@mosaic/db':
         specifier: workspace:^
         version: link:../../packages/db