* schedule: add `paseo schedule update` to edit schedules in place
Editing a schedule today requires delete+recreate, losing run history
and the schedule id. This adds an additive RPC and CLI command that
patches name, prompt, cadence, new-agent target fields, max-runs, and
expires-in without touching runs or in-flight executions. nextRunAt is
recomputed only when the cadence actually changes.
* schedule(cli): share cadence flag parser between create and update
Both paths were turning --every/--cron into a ScheduleCadence with
near-identical code. Extract `parseCadenceFromFlags` so the literals
and the exclusivity check live in one place; create wraps it to
require a value, update lets it stay optional.