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/drizzle.config.ts

@@ -0,0 +1,10 @@
+import { defineConfig } from "drizzle-kit";
+
+export default defineConfig({
+  schema: "./src/schema/index.ts",
+  out: "./src/migrations",
+  dialect: "postgresql",
+  dbCredentials: {
+    url: process.env.DATABASE_URL!,
+  },
+});