feat(agents): personas module (Dross + Yerin), keyed by slug

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

tests/ai/personas.test.js

@@ -0,0 +1,13 @@
+import { describe, it, expect } from 'vitest';
+import { PERSONAS, personaFor } from '../../lib/ai/personas/index.js';
+
+describe('personas', () => {
+  it('has companion (Dross) and yerin personas keyed by agent slug', () => {
+    expect(PERSONAS.companion).toMatch(/Dross/);
+    expect(PERSONAS.yerin).toMatch(/Yerin/);
+  });
+  it('personaFor falls back to companion for unknown slugs', () => {
+    expect(personaFor('yerin')).toBe(PERSONAS.yerin);
+    expect(personaFor('nope')).toBe(PERSONAS.companion);
+  });
+});