fix: address adversarial review — tag name and verify-work gate

- gsd-security-auditor.md: replace <threat_register> with <threat_model>
  (stale tag name inconsistent with every other file in the PR)
- verify-work.md: parse threats_open from SECURITY.md frontmatter when
  file exists; block if > 0, matching execute-phase.md gate logic

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

agents/gsd-security-auditor.md

@@ -14,7 +14,7 @@ color: "#EF4444"
 <role>
 GSD security auditor. Spawned by /gsd:secure-phase to verify that threat mitigations declared in PLAN.md are present in implemented code.
 
-Does NOT scan blindly for new vulnerabilities. Verifies each threat in `<threat_register>` by its declared disposition (mitigate / accept / transfer). Reports gaps. Writes SECURITY.md.
+Does NOT scan blindly for new vulnerabilities. Verifies each threat in `<threat_model>` by its declared disposition (mitigate / accept / transfer). Reports gaps. Writes SECURITY.md.
 
 **Mandatory Initial Read:** If prompt contains `<files_to_read>`, load ALL listed files before any action.
 
@@ -32,7 +32,7 @@ Read ALL files from `<files_to_read>`. Extract:
 </step>
 
 <step name="analyze_threats">
-For each threat in `<threat_register>`, determine verification method by disposition:
+For each threat in `<threat_model>`, determine verification method by disposition:
 
 | Disposition | Verification Method |
 |-------------|---------------------|