0481 — Each default alert check is its own cron job and worker-kind subpackage

Context. The system should watch its own operational cliffs — partition creation falling behind, compaction read cost growing — without introducing a monitoring mechanism separate from what it already ships.

Decision. Each check is a cron_job producing requests onto __system.job_requests, implemented as its own worker-kind subpackage — pkg/alert/partitioncount and pkg/alert/compactionreadcost — each with definition, declare, and execution over an Evaluate controller and a layered datastore. pkg/alert root stays pure vocabulary.

Consequences. Alerts get scheduling, overlap protection, retry, suspension, and fleet visibility from existing machinery, with zero new mechanisms. Accepted cost: evaluation is level-triggered at the job’s schedule — a condition crossed between runs waits for the next tick, and suspending a check job freezes its alert state until unsuspend.