Add database package with Drizzle schema

Postgres schema via Drizzle ORM for agents, goals, issues, projects,
and activity log tables. Includes migration runner, seed script,
and Drizzle config.

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

packages/db/src/index.ts

@@ -0,0 +1,2 @@
+export { createDb, type Db } from "./client.js";
+export * from "./schema/index.js";