Messages from /hooks/agent containing SECURITY NOTICE blocks and
<<<EXTERNAL_UNTRUSTED_CONTENT>>> delimiters are now cleaned up.
Only the actual user content is displayed, with a small webhook
badge indicator showing the message originated from a webhook.
Closes feedback #54
System events (heartbeats, cron triggers, webhooks, channel events)
now render as subtle inline notifications instead of full user bubbles.
Detection based on content patterns ([EVENT], [cron:], [HEARTBEAT], etc.).