feat(agents): conversations.findOrCreateGlobal for space-less agents

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/db/conversations_global.test.js

@@ -0,0 +1,19 @@
+import { describe, it, expect, beforeAll } from 'vitest';
+import { resetDb } from '../helpers/db.js';
+import { migrateUp } from '../../lib/db/migrate.js';
+import * as agents from '../../lib/db/repos/agents.js';
+import * as conversations from '../../lib/db/repos/conversations.js';
+
+const owner = { kind: 'user', id: null };
+beforeAll(async () => { await resetDb(); await migrateUp(); });
+
+describe('findOrCreateGlobal', () => {
+  it('creates one space-less open conversation and reuses it', async () => {
+    const y = await agents.getBySlug('yerin');           // seeded by 011_yerin.sql
+    const c1 = await conversations.findOrCreateGlobal(y.id, owner);
+    expect(c1.space_id).toBeNull();
+    expect(c1.agent_id).toBe(y.id);
+    const c2 = await conversations.findOrCreateGlobal(y.id, owner);
+    expect(c2.id).toBe(c1.id);
+  });
+});