Two CLI polish fixes found during a full command sweep:
- Validate --provider against the known provider set (report, status, today,
month, export, optimize, compare, models). An unknown provider previously
produced a silently empty report; it now exits 1 with a clear message
listing valid values, matching how --period and --format already behave.
Provider names are exposed via a lazy allProviderNames() helper so importing
the providers module never depends on every provider object being defined.
- Make the non-interactive report/today/month render deterministic: yield a
tick so ink flushes the one-shot frame to stdout before unmounting, instead
of unmounting synchronously which could race the flush and drop the output.
Adds CLI provider-validation tests (rejection, valid name, all sentinel, and
a drift guard that allProviderNames covers every loadable provider).
