Add validate_cycle_result() to loop_guard.py that runs before each cycle:
1. Age check — removes cycle_result.json if older than 2× CYCLE_DURATION
2. Issue check — removes cycle_result.json if referenced issue is closed
3. Graceful degradation — keeps file when Gitea API is unreachable
Also adds _load_cycle_result() and _is_issue_open() helpers, plus 15 unit
tests covering all validation paths.
Fixes#661
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
