Refactor idle-notification in terms of emit-event system

[timvisher-dd]

Replace direct idle-notification-start/cancel calls with event subscriptions: turn-complete starts the timer, clean-up cancels it. This removes the coupling between the idle notification feature and the specific call sites that emit these events, letting the event system handle the wiring instead.

Direct cancel calls remain at user-input boundaries (--insert-to-shell-buffer, agent-shell-queue-request) since those are programmatic inputs that don't trigger post-command-hook.

Also fixes the pre-existing agent-shell-mode-hook-subscriptions-survive-state-init test failure by stubbing agent-shell--handle (the test only cares about subscription survival, not the initialization pipeline) and relaxing the subscription count assertion to accommodate additional internal subscriptions.

Checklist

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