When a plan reaches a terminal state (all items done or blocked) in
interactive mode, automatically exit plan mode and return to normal
prompt.
Changes:
- Add Agent::is_plan_terminal() method to check if plan is complete
- Add check_and_exit_plan_mode_if_terminal() helper in interactive.rs
- Call the helper after each execute_user_input() to detect completion
Fixes issue where plan mode prompt ' >> ' persisted after plan completion.
19162b1fe6