Feature request: out-of-band notification when an approval is pending

## Environment

aac 0.11.0 (3b000d15), macOS 26.5.1 (arm64).

## Summary

The human-in-the-loop model depends on the operator noticing a pending fingerprint confirmation or credential approval, but `aac listen` is a TUI that in real use sits in a background terminal (or another desktop/space) while the operator works elsewhere -- e.g. driving an AI coding agent that issues `aac run` requests. Today there is no signal at all that a request is waiting; combined with prompt queueing (#139) this makes timeouts the common case rather than the exception.

## Suggestion

Emit an out-of-band notification when a request transitions to pending:

- Desktop notification (osascript on macOS, notify-send on Linux, toast on Windows), and/or
- terminal bell (BEL) as a minimal fallback, and/or
- a `--notify-cmd ` hook so operators can wire up anything (Pushover, Slack, etc.).

Happy to provide more detail on our setup if useful.