hermes - 💡(How to fix) Fix fix(cron): config migration (23 to 24) may silently clear cron/jobs.json

Bug Description

After running hermes update (v2026.5.29, 118 commits), which migrated config version from 23 to 24, the file ~/.hermes/cron/jobs.json was found empty — all 17 scheduled cron jobs were lost silently.

Impact

• All cron jobs stopped working
• User only noticed hours later when expected reports didn't arrive
• No automatic alert or restoration mechanism
• The pre-update snapshot (state-snapshots/YYYYMMDD-HHMMSS-pre-update/cron/jobs.json) retained the data, but no auto-restore occurred

Steps to Reproduce

1. Have ~17 cron jobs configured
2. Run hermes update with config version migration (23 to 24)
3. Observe that cron/jobs.json is empty after the update
4. Cron ticker restarts with 0 jobs

Root Cause Analysis

The file ~/.hermes/cron/jobs.json exists after the update but is empty or truncated. The config migration code path may be rewriting the file without preserving existing jobs.

Previous related fix: commit 2a6cbf52d — but this scenario may not raise because the file is valid JSON, just empty.

Expected Behavior

• Config migration should preserve existing cron jobs
• OR: update should warn before discarding jobs
• OR: auto-restore from pre-update snapshot

Environment

• Hermes Agent v0.15.1 (2026.5.29)
• Linux
• Python 3.11.2
• Cron storage: jobs.json

Workaround

Restore from ~/.hermes/state-snapshots/YYYYMMDD-HHMMSS-pre-update/cron/jobs.json.

Suggested Fix

1. Migration handler preserving existing jobs
2. Post-update integrity check comparing with snapshot
3. Warning during update if jobs will be discarded