[dcellison]

Closes #1819

Summary

The conversation introduction (the block above the first message) now uses plain language:

	Before	After
Group channel label	Canonical conversation	Channel
Direct message label	Direct conversation	Direct message
Notification feed label	Durable notification feed	Notifications
Group channel sentence	Messages below come from 's durable conversation history across every connected client.	Everyone in this channel sees the same messages, in every browser and app.
Agent direct message sentence	(same as the group channel sentence)	Your conversation with is the same in every browser and app.

The human direct message, archived, and notification sentences are unchanged.

Testing

• npm run typecheck clean.
• npx vitest run: 289 passed. The notification and agent direct message tests check the new labels and sentences, and the channel lifecycle test now checks the group channel label and sentence.
• npm run verify-generated clean.