ci: add reboot and verification steps to nightly runner cleanup (drive-by clean up fast push) (#2708)

## Summary
- Add reboot step after cleanup script to ensure fresh runner state
- Add verification job that runs on all CI runners to check disk space
post-reboot
- Verification job runs even if cleanup fails (since reboot kills the
runner mid-job)

## Test plan
- Verify the workflow structure is valid by checking GitHub Actions UI
- Monitor next nightly run to ensure reboot completes successfully
- Confirm verification job runs and reports disk space

🤖 Generated with [Claude Code](https://claude.com/claude-code)
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