Add logging when group chat classifier decides to engage

When the shouldEngageInGroup classifier returns "yes", there's no log indicating this decision. We only see logs for:

• "Engaging (explicit mention of 'ava')"
• "Engaging (reply to Ava's message)"
• "Skipping group message (pre-filter said no)"

But when the classifier says yes, it silently proceeds. This makes debugging engagement issues difficult.

Fix: Add a log line like putText "Engaging (classifier said yes)" after the classifier returns True in the else branch around line 1144 of Omni/Agent/Telegram.hs.

Context: This came up investigating why Ava responded to Kate's group message at 20:16 on Jan 27 - couldn't tell from logs why the classifier decided to engage.