fix: prevent duplicate review_all runs via reason-based dedup (#848)

Two problems:
1. Schedule was every 20 min but review_all cycles take 35 min,
   causing overlapping triggers that fill both slots
2. Trigger server only deduped by issue number, not by reason,
   so two review_all runs could stack up

Fixes:
- Change schedule from */20 to 0,45 (every 45 min)
- Add reason-based dedup in trigger-server.ts: reject 409 if a
  non-issue run with the same reason is already in progress

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.github/workflows/security.yml

@@ -4,8 +4,8 @@ on:
   issues:
     types: [opened, reopened]
   schedule:
-    # Consolidated review + scan — every 20 min
-    - cron: '*/20 * * * *'
+    # Consolidated review + scan — every 45 min (must exceed 35-min cycle timeout)
+    - cron: '0,45 * * * *'
   workflow_dispatch:
     inputs:
       mode: