action_interrupt

Handle escape key.

Priority order:

1. If modal screen is active, dismiss it
2. If completion popup is open, dismiss it
3. If input is in command/shell mode, exit to normal mode
4. If shell command is running, kill it
5. If approval menu is active, reject it
6. If ask-user menu is active, cancel it
7. If queued messages exist, pop the last one (LIFO)
8. If agent is running, interrupt it