feat(db): @mosaic/db — Drizzle schema, PG connection, migrations

Implements P0-003: shared database package with Drizzle ORM.

Schema: 13 tables across 3 domains:
- Auth (BetterAuth-compatible): users, sessions, accounts, verifications
- Brain: projects, missions, tasks, events, agents, tickets, appreciations
- Chat: conversations, messages

Features:
- createDb() factory with pool cleanup via DbHandle.close()
- runMigrations() with try/finally for connection safety
- drizzle-kit config for generate/push/migrate/studio
- Full FK relationships and indexes on query-hot columns
- ESM package with NodeNext resolution

Verified: schema pushed to PG 17, CRUD round-trip tested.

Closes #3

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

packages/db/drizzle.config.ts

@@ -0,0 +1,10 @@
+import { defineConfig } from 'drizzle-kit';
+
+export default defineConfig({
+  schema: './src/schema.ts',
+  out: './drizzle',
+  dialect: 'postgresql',
+  dbCredentials: {
+    url: process.env['DATABASE_URL'] ?? 'postgresql://mosaic:mosaic@localhost:5433/mosaic',
+  },
+});