Add hook for permission prompt

[raine]

Describe the feature or problem you'd like to solve

In Claude Code, you have specific hook that fires when user is prompted for a permission, https://code.claude.com/docs/en/hooks#notification

Proposed solution

Add a similar hook to Copilot CLI. This will allow creating integrations that notify user when Copilot is asking for users permission to run some tool etc.

Example prompts or workflows

I want to support Copilot CLI in a tool, and the use case is that user is notified if Copilot CLI is waiting for user input. With the current hook setup, this does not appear possible.

Additional context

No response

[huntj88]

I also needed this behavior. This is possible with --acp if you need a workaround

[maxbeizer]

hitting this with cmux + Copilot CLI.

cmux has first-class Claude Code support via a wrapper that injects Notification hooks automatically. For Copilot
CLI, I've wired up postToolUse to catch ask_user and fire cmux notify, which works — but the "Plan Ready for
Review" prompt and permission prompts don't fire any hook at all.

Debug log shows a ~3 min gap between the last postToolUse and exit_plan_mode — no event fires while the plan
review UI is blocking.

What's needed: A hook (like Claude Code's Notification) that fires for all blocking user prompts — plan review,
permissions, ask_user, etc. The current preToolUse/postToolUse hooks don't cover built-in UI prompts.

Additional data point: preToolUse doesn't fire for ask_user at all — only postToolUse does. That may be a
separate bug.