0658 — Built-in alert config names the alert, not its scheduled work

Context. RegisterSystemConfig.PartitionCount, CompactionReadCost, and WorkerLiveness named conditions without saying what was configured. At the call site, PartitionCount looked like a system capacity setting. Their *JobConfig types and Expression fields recovered the missing meaning by exposing the internal scheduled-work mechanism.

Decision. The fields become PartitionCountAlert, CompactionReadCostAlert, and WorkerLivenessAlert. Their types become the matching *AlertConfig, and Expression becomes ScheduleExpression. The flat fields remain: they keep every built-in alert directly discoverable from RegisterSystemConfig without adding a grouping type that owns no new concept.

Consequences. A declaration reads as the thing configured and the precise setting applied to it. These pre-v1 names replace the old public names without compatibility aliases; implementation-local scheduled-work names are unchanged.