Notify participants when an agent finishes

Nate note: Old ace had this: glow, etc. Review that before stating this.

Notify participants when a run finishes or fails, or an agent question needs an answer.

Dogfooding core/QOL: #5. Migrated from TODO.md — During dogfooding.

Acceptance

Surface attention while another channel/window is active, with a link to the relevant chat.
Distinguish finished, failed and waiting-for-answer states; coordinate questions with Let agents ask participants questions in the chat #21.
Reconnect without losing pending attention or repeatedly notifying about the same outcome.

Done in #255 (apps/app/src/attention.ts).

When it fires: a run you invoked ends while you aren't watching that channel.
How it shows: the window glows when Ace is focused elsewhere, or you get one desktop notification ("Agent finished" or "Agent run failed"). The channel's row stays marked until you open it.
What it skips: stopped runs, since you did that yourself.
No repeats: what you've seen and been told is persisted per host and user, so reconnecting doesn't notify you again or lose pending attention.

Waiting-for-an-answer attention belongs to agent questions, so it's tracked with #21.