fix: extend step 8 approval-bias carve-out to include infra files (step 3c), fix count

- Step 8 now explicitly exempts infrastructure file findings (step 3c) from
  the "bias toward APPROVE" guidance, preventing the original failure mode
- Fix investigation summary: "Five" → "Six" structural gaps

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

docs/investigation-685-reviewer-approved-destructive-compose.md

@@ -8,7 +8,7 @@
 
 The reviewer agent approved PR #683 in ~1 minute without flagging that it
 contained a destructive rewrite of `docker-compose.yml` — dropping named
-volumes, bind mounts, env vars, restart policy, and security options. Five
+volumes, bind mounts, env vars, restart policy, and security options. Six
 structural gaps in the review pipeline allowed this to pass.
 
 ## Root causes