The edit_schedule MCP tool accepts interval_seconds as Option<u64>
(positive values only), mapping None→leave-alone and Some(v)→set.
The handler wraps this as .map(Some) before dispatching, so there
is no way to express Some(None) = clear via the agent surface.
The old doc said 'Clearing interval_seconds to null flips
recurring → one-shot' which is only true for the dashboard
PATCH endpoint (which uses the full double-Option form).
Corrected: interval_seconds is positive-only via the MCP tool;
toggling recurring→one-shot is operator-only via the dashboard.
