Feature request: create scheduled maintenance incidents (--kind, --scheduled-for/--scheduled-until)

[minkimipt]

Use case

Scheduling a planned maintenance (a phased extension upgrade across a fleet) from a script, the same way the UI's Plan Maintenance dialog does.

What's missing

incidents create / incidents update can't set the incident kind or the maintenance window. The API client already reads these fields in internal/api/client.go:

Kind           string  `json:"kind"`
ScheduledFor   *string `json:"scheduled_for"`
ScheduledUntil *string `json:"scheduled_until"`

but none of them are exposed as flags, so every incident the CLI creates is a normal incident starting now.

Why it matters

A maintenance has to be created as a scheduled incident with a window — that's what the status page renders as "Scheduled maintenance" and what subscribers are notified about. Without it, the create step can only be done in the UI, and the rest of the lifecycle (status-page events, which the CLI now supports) has nothing to attach to until someone clicks through the dialog.

Proposed

rootly incidents create --title="..." --kind=scheduled \
  --scheduled-for=2026-10-05T08:00:00Z --scheduled-until=2026-10-26T08:00:00Z \
  --services=database
rootly incidents update <id> --scheduled-until=2026-10-27T08:00:00Z

RFC3339 timestamps, consistent with status-pages events create --started-at.