[app-cli] Emit a lifecycle event on user interrupt (Ctrl+C currently writes nothing to events.jsonl)

Observed on amplifier 2026.07.06-7ec5dcd (scripted PTY, twice — once during a long bash tool call, once during a long streaming response): sending \x03 mid-turn wrote zero records to events.jsonl — no cancel:*, no prompt:complete. (In our scripted-PTY environment the turn also ran to completion, so this may interact with prompt_toolkit key handling; the on-disk silence is the part this issue is about.)

Problem. External consumers of the event log cannot distinguish "turn interrupted by the user" from "turn still running". For tooling that shows busy/idle state per session, an interrupted turn leaves a stale busy indicator until the next turn completes.

Ask. Emit a lifecycle record when a turn is interrupted/cancelled — either a cancel:* event or a prompt:complete with a status: cancelled field — so the turn's open/closed state is always recoverable from the log.

Start by locating the app-cli handling for Ctrl+C or turn cancellation and the code that writes lifecycle records to events.jsonl. Reproduce an interrupt during a long bash call or streaming response, then ensure the log contains a cancellation or cancelled completion record so the turn's open/closed state is recoverable.