Allow plugin monitors to opt out of the exit confirmation prompt

Summary

Plugin monitors (monitors/monitors.json) are session-scoped: they start with the session and stop when it ends. Exiting a session with an active monitor currently triggers a confirmation prompt:

  Background work is running
  The following will stop when you exit:

  shell · <monitor description>

  ❯ 1. Exit anyway
    2. Stay

For monitors, the monitor stopping is the expected behavior on exit. The prompt adds a friction step on every session exit when a plugin includes an always-on monitor.

Proposal

One of:

1. Per-monitor silentExit: true field on entries in monitors/monitors.json — opts that monitor out of the prompt.
2. Global suppressBackgroundExitConfirmation setting — less ideal because it would also affect Bash background tasks.

Option 1 is preferred: the plugin author declares intent at config time. Other long-running work keeps the existing prompt.

Context

Use case: a plugin runs a mailbox-watcher monitor that delivers cross-agent messages to Claude as monitor lines. The monitor's lifetime is exactly the session's lifetime — nothing to preserve on exit. The confirmation is the only friction point.

Related

• #18544 — different problem (completion-notification token waste) but in the same area.