fix(#2119): single SECURITY.md writer — auditor is return-only (#2154)

* fix(2119): single SECURITY.md writer — auditor is return-only

The gsd-security-auditor held Write/Edit and was instructed to write
SECURITY.md (no <N>- prefix, no template frontmatter), while the
orchestrator's Step 6 also wrote the correct padded <N>-SECURITY.md
from templates/SECURITY.md. Two writers, two naming conventions, two
shapes — the auditor's unprefixed file was invisible to the workflow's
*-SECURITY.md glob detector and unparseable for the threats_open gate.

Fix (option 1 from the issue): make the auditor return-only.
- Remove Write/Edit from auditor's tools
- Rewrite all 'Write SECURITY.md' instructions to 'Return structured
  verdict' with threats_open count
- Add explicit constraint in workflow Step 5 spawn prompt
- Update existing test (was asserting Write in tools — now asserts absence)
- Add new regression test for single-writer contract
- Update docs/AGENTS.md stale Tools/Produces rows
- Regenerate golden fixtures + agent size baseline

* docs(changeset): backfill PR number (#2154)

* chore(#2119): regenerate pi/qwen golden fixtures after next merge

The single-writer change edits gsd-core/workflows/secure-phase.md and
agents/gsd-security-auditor.md; pi.json (added on next) and qwen.json (merge
straggler) were the only runtime fixtures still holding pre-change hashes for
those files. All other runtimes already reflect the change. Regenerated via
the sanctioned gen-golden-install-parity script.

* merge origin/next — regenerate goldens + baseline for merged state

* fix slash-command syntax: /gsd-secure-phase → /gsd:secure-phase (#2154 CI fix)

gsd-core/workflows/secure-phase.md

@@ -108,7 +108,7 @@ Agent(
     "<files_to_read>{PLAN, SUMMARY, impl files, SECURITY.md}</files_to_read>" +
     "<threat_register>{threat register}</threat_register>" +
     "<config>asvs_level: {SECURITY_ASVS}, block_on: {SECURITY_BLOCK_ON}</config>" +
-    "<constraints>Never modify implementation files. Verify mitigations exist — do not scan for new threats. Escalate implementation gaps.</constraints>" +
+    "<constraints>Never modify implementation files. Verify mitigations exist — do not scan for new threats. Escalate implementation gaps. Return a structured verdict only — do NOT write SECURITY.md (the orchestrator owns the file write).</constraints>" +
     "${AGENT_SKILLS_AUDITOR}",
   subagent_type="gsd-security-auditor",
   model="{AUDITOR_MODEL}",