Queue a purge of the device's 'pipeline' queue

Destroy every execution waiting on the device's pipeline queue. Irreversible. Intended for a device whose runner has been down long enough that the queued executions are stale. Writes a command file into the device's container; configsync picks it up on its next sync and purges, once. This returns when the command is written, not when it has run — there is no device-to-cloud return channel before DEV-6016, the same reason edge deployment is fire-and-forget. What actually happened shows up in the device's log and in the next queue-watchdog reading.