Document that Git Branch cron registrations are disabled by default

The Deploy cron reference says:

> Cron jobs run on production and git branch timelines.

However, cron registrations on non-production timelines are disabled by default. This is not currently documented, which makes a correctly registered preview cron look broken.

Please update `deploy/reference/cron.md` to explain:

- Production timeline cron registrations are enabled by default.
- Git Branch timeline cron registrations are disabled by default.
- To enable a branch registration, open the app's **Cron** tab, select **View Details**, open the **Enable/Disable** dropdown, and choose **Enable for Git Branch/**.
- Once enabled, the job runs against that timeline's active revision.

This came from support ticket `TICKET-20260725-22913`, where the customer's cron jobs were registered on Production and multiple Git Branch timelines, but only Production was enabled.