Measuring a worker that does not care
Service level objectives assume the worker wants to meet them. Familiars, and cats in particular, do not. A ward-patrol cat with 71% uptime is not failing; it is expressing a preference.
Start with what the familiar's customers actually need. For a courier owl, that is delivery latency, measured from dispatch to arrival. For a ward patrol, it is coverage: how much of the night the perimeter was watched, by anyone. Set the objective on that, not on the familiar's own heartbeat.
Then leave room for mood. An error budget a familiar can spend on naps is an error budget that will be spent. Track it, alert on trends rather than single bad hours, and keep snacks within reach of the on-call wizard.