feat: add RHIZA_SYNC_SCHEDULE variable to override default sync cron

[Copilot]

Summary

Adds RHIZA_SYNC_SCHEDULE Makefile variable so users can override the default weekly sync cron (0 0 * * 1) without their change being clobbered on every make sync.

Changes

• Add RHIZA_SYNC_SCHEDULE ?= 0 0 * * 1 to .rhiza/rhiza.mk
• Add _apply-sync-schedule target that patches .github/workflows/rhiza_sync.yml via sed after uvx rhiza sync . completes but before post-sync hooks
• Use portable sed -i.bak && rm .bak for macOS/Linux compatibility
• Document the override in docs/CUSTOMIZATION.md with cron examples
• Add 8 tests covering default value, env/Makefile override, patching, skip-when-default, missing-file, and integration with sync target

Usage — set before the include line in your root Makefile:

RHIZA_SYNC_SCHEDULE = 0 9 * * 1-5   # weekdays at 9 AM UTC

include .rhiza/rhiza.mk

GitLab CI schedules are configured in the UI, so this variable only affects GitHub Actions.

Testing

• make test passes locally
• make fmt has been run
• New tests added (or explain why not needed)

Checklist

• Commit messages follow the Conventional Commits format
• Documentation updated if behaviour changed
• make deptry passes (no unused or missing dependencies)

[HarryCampion]

This is neat @markrichardson. Is it overridden via an env var? Or does user need to override the rhiza.mk?

[HarryCampion]

This is neat @markrichardson. Is it overridden via an env var? Or does user need to override the rhiza.mk?

Ignore this I see it tested. Looks great

[tschm]

Rhiza is no longer overwriting files (you have modified) with a hammer. So, the patching exercise should be obsolete. Let's first verify that this is indeed correct and if not we should do as sketched here?