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/schema/index.ts

@@ -0,0 +1,5 @@
+export { agents } from "./agents.js";
+export { projects } from "./projects.js";
+export { goals } from "./goals.js";
+export { issues } from "./issues.js";
+export { activityLog } from "./activity_log.js";