Under the hood
A background worker can now be retired purely for running a stale CLI version, with new retirement causes recorded.
The background-agent worker manager can now retire a worker purely because its CLI version is stale (this.isVersionStale), even when the worker isn't idle, as long as it isn't running session_cron, isn't a routine, has no in-flight tasks or queue, and its last update predates a threshold (BMe). The tengu_bg_retired telemetry event gains a cause field with values including abandoned-stale, settled, idle-prompt, and empty-idle, plus a worker_cli_version field.