fix: resolve review round 3 findings

- reviewer, auditor: add qa-checklist to skills (both produce envelopes)
- worker-protocol: annotate ac_coverage as optional in envelope template
- message-schema: document that security_findings.high is non-blocking
- qa-checklist: reframe plan_result has_blockers as intent confirmation

agents/reviewer.md

@@ -9,6 +9,7 @@ maxTurns: 20
 skills:
   - conventions
   - message-schema
+  - qa-checklist
   - project
 ---