feat: add status dashboard for live agent monitoring

Dedicated Discord channel shows per-group agent status with live
elapsed time. Single message is edited every 10s instead of spamming.

- GroupQueue tracks startedAt timestamps and exposes getStatuses()
- Channel interface gets editMessage/sendAndTrack for message editing
- STATUS_CHANNEL_ID env var to configure the dashboard channel
- Shows processing/idle/waiting/inactive per registered group

src/channels/discord.test.ts

@@ -12,6 +12,7 @@ vi.mock('../env.js', () => ({ readEnvFile: vi.fn(() => ({})) }));
 vi.mock('../config.js', () => ({
   ASSISTANT_NAME: 'Andy',
   TRIGGER_PATTERN: /^@Andy\b/i,
+  DATA_DIR: '/tmp/nanoclaw-test-data',
 }));
 
 // Mock logger