Add engagement-only conversation mode

tests/config.test.ts

@@ -80,7 +80,10 @@ describe('ConfigSchema', () => {
       }
     });
 
-    it('accepts r4r and manual behaviour while enforcing the autonomous turn limit', () => {
+    it('accepts engagement, r4r, and manual behaviour while enforcing the autonomous turn limit', () => {
+      expect(ConfigSchema.safeParse(minimalConfig({
+        reddit: { subreddits: [{ name: 'r4r', conversation: { mode: 'engagement' } }] },
+      })).success).toBe(true);
       expect(ConfigSchema.safeParse(minimalConfig({
         reddit: { subreddits: [{ name: 'r4r', conversation: { mode: 'r4r', maxAutomatedTurns: 12 } }] },
       })).success).toBe(true);