fix(status): do not go quiet on a host whose units predate run records

[vishr]

What this changes

Found while preparing the first release that carries #156, and it should not ship as it stands.

Run records are written by the generated unit, so a host still running units from before that change has none. ob status had stopped querying systemd's Result and ExecMainStatus entirely and derived its verdict only from records. The consequence is that upgrading ob on the workstation — no deploy, nothing changed on the host — turns a failing scheduled job into a clean report and an exit 0, until someone happens to run ob schedule apply.

That is the exact direction the feature exists to close, arriving as a side effect of installing it.

With no records at all, systemd's retained result is the only evidence there is, so status uses it and says what it is looking at:

schedule nightly-dump last run failed: timeout (exit 143); no run record — run `ob schedule apply` so runs are recorded ⚠

Once records exist they remain the verdict, and a stale systemd result raises nothing.

Why this is correct

• The fallback fires only when a job has no records at all, so it cannot produce the contradictory pair of answers that made the record the sole verdict in the first place.
• TestScheduleStatusFallsBackToSystemdWhenNoRecordsExist covers the pre-upgrade host; TestScheduleStatusPrefersRecordsOverSystemdResult pins a retained Result=timeout losing to a success record.
• The scheduling guide's upgrade section now states the transition rather than leaving it to be discovered.
• just check and just lint pass.

Effect on the safety envelope

Restores a divergence signal that #156 removed for hosts that have not been reconciled yet. It only adds an issue where status was silent; it cannot clear one.

Checklist

• just check passes locally.
• Tests cover the new behaviour, including the failure paths.
• Generated documentation is current (just check verifies this).
• I have accepted the CLA, or will when the bot asks on my first pull request.

https://claude.ai/code/session_013MqwrF5NA179khDtEQdib5