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)

tests/fixtures/golden-install-parity/zcode.json

@@ -29,7 +29,7 @@
   "agents/gsd-project-researcher.md": "f572892f138734ff",
   "agents/gsd-research-synthesizer.md": "29949bf3f049a8f1",
   "agents/gsd-roadmapper.md": "840ac933e3b094f9",
-  "agents/gsd-security-auditor.md": "b7202c44366697dd",
+  "agents/gsd-security-auditor.md": "4b86fa11ebda981e",
   "agents/gsd-ui-auditor.md": "76f446c50edf81f8",
   "agents/gsd-ui-checker.md": "15949ccab982b71c",
   "agents/gsd-ui-researcher.md": "0e8ec8509d476904",
@@ -358,7 +358,7 @@
   "gsd-core/workflows/resume-project.md": "f28da1200e4545f4",
   "gsd-core/workflows/review.md": "ad7c0f372ed986ae",
   "gsd-core/workflows/scan.md": "003883d71c37da7d",
-  "gsd-core/workflows/secure-phase.md": "29fc6b62c5c5dc62",
+  "gsd-core/workflows/secure-phase.md": "22ab3bb5da494ea7",
   "gsd-core/workflows/session-report.md": "2e5b1205324ddefa",
   "gsd-core/workflows/settings-advanced.md": "414db4dbea97ba44",
   "gsd-core/workflows/settings-integrations.md": "70515c5838fb9826",