Document that Git Branch cron registrations are disabled by default

[avocet-bot]

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.