Three bugs found during first CI run after enabling :exclude:
- Derive old filter (:/subfolder) when state has no josh_filter stored
(pre-v1.2 upgrade path)
- Detect unrelated histories in forward_sync() and fall back to
reconcile_filter_change() instead of creating a useless conflict PR
- Skip state update on conflict result (prevents storing wrong filter
and mono SHA that blocks retries)
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>