fix(cron): normalize interval run times to UTC

[Achordchan]

Change Summary

Interval schedules currently format non-UTC clock times with a literal +00:00 suffix. For example, advancing 2099-01-01T09:00:00+08:00 by one hour produces 10:00+00:00 instead of 02:00+00:00, shifting the next run by eight hours. This also affects persisted offset-bearing next_run_at values advanced by JobManager.record_result().

Convert interval results to UTC before formatting in both scheduling paths. Preserve the existing interpretation of naive interval base times as UTC. No new configuration or dependencies.

Tests cover positive/negative offsets, UTC, naive inputs and overdue fast-forwarding. The original implementation fails the offset regression cases. A temporary-directory check through JobManager and the JSON repository confirms the correct next run is persisted and reloaded.

Validation: python -m pytest tests/cron -q (106 passed), changed-file pre-commit checks, and git diff --check. Full-repository pre-commit was also attempted; it reports existing lint/style failures, including .dev_scripts/webui/oss_distribution.py:140 and ms_agent/config/skills_manager.py:61. Unrelated automatic formatting was excluded from this patch.

Prepared and tested with Codex assistance.

Related issue number

None; found while checking timezone handling in the cron parser and scheduler. This does not overlap #997's one-shot timestamp validation.

Checklist

• The pull request title is a good summary of the changes - it will be used in the changelog
• Unit tests for the changes exist
• Run pre-commit install and pre-commit run --all-files before git commit, and passed lint check. Both ran; baseline failures are described above.
• Documentation reflects the changes where applicable (no new command or configuration)