Local routines: support custom cron expressions in the UI

Feature Request

Summary

Local routines in the Claude desktop app currently only support preset schedule intervals (e.g. Daily, Weekly). There is no way to enter a custom cron expression through the UI, even though the remote routines API already supports arbitrary cron expressions via the cron_expression field.

Current behavior

When creating a local routine, the schedule picker only offers fixed options (Daily, Weekly, etc.). There is no free-form cron input.

Desired behavior

Add a custom cron expression input option to the local routine schedule UI, equivalent to what the remote RemoteTrigger API already supports. For example, a user should be able to enter 0 10 23 4 * to fire once on April 23rd at 10 AM, or 0 9 * * 1-5 for weekdays only.

Why this matters

• Without custom cron support, one-time or non-standard schedules (e.g. "only on the 1st of each month", "once next Tuesday") are impossible via local routines.
• As a workaround, users have to add a date guard inside the routine prompt itself (e.g. "check today's date and exit if it's not April 23") — which is a poor UX and wastes a session startup just to do nothing.
• Remote routines already support this via cron_expression in the API, so the backend capability exists. The gap is purely in the local routine UI.

Suggested UI

A "Custom" option in the schedule picker that reveals a cron expression text field, with a human-readable preview (e.g. 0 10 23 4 * → "At 10:00 AM, on day 23 of April").

Also consider a "Run once" option that auto-disables the routine after the first firing, which would cover the most common one-shot use case without requiring users to know cron syntax.