Wire the orphaned nuncio-cal engine into nunciod as a real, authenticated
gRPC vertical mirroring Mail: add service Calendar (Sync/ListEvents/GetEvent)
to nuncio.proto with a regenerated descriptor golden, connect_calendar in
nuncio-proto's client, calendar_sync::sync_with_backend plus
CalendarEngineOverrides/CalendarGrpcService mounted behind its own
BearerAuthInterceptor in nunciod::grpc, and real `cal sync`/`cal list` gRPC
clients in nuncio-cli replacing the fabricated CLI output. ListEvents/GetEvent
always read genuinely persisted data; production Sync returns an honest error
since there is no persisted per-account CalDAV configuration yet, and only an
injected MockCalendarBackend exercises a working Sync. Adds unit, daemon
integration, and offline full-daemon E2E tests, including an
unauthenticated-call-rejected assertion for the new service.
Refs #185
James Maes