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/client.ts

@@ -0,0 +1,10 @@
+import { drizzle } from "drizzle-orm/postgres-js";
+import postgres from "postgres";
+import * as schema from "./schema/index.js";
+
+export function createDb(url: string) {
+  const sql = postgres(url);
+  return drizzle(sql, { schema });
+}
+
+export type Db = ReturnType<typeof createDb>;