Cancel the countdown on any terminal input, not just "editable" keys.
Arrow keys and other navigation are escape sequences that
isEditableInput() explicitly rejected, so navigating the handoff draft
didn't stop the auto-submit timer.

Extract parseAndSetModel() into model-utils.ts to deduplicate the
model-parsing logic shared between the agent_end handler in index.ts and
the /handoff command. The shared helper also wraps pi.setModel() in
try/catch, surfacing failures as user-visible warnings instead of
unhandled promise rejections.