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)

docs/AGENTS.md

@@ -479,10 +479,10 @@ Communication style, decision patterns, debugging approach, UX preferences, vend
 |----------|-------|
 | **Spawned by** | `/gsd-secure-phase` |
 | **Parallelism** | Single instance |
-| **Tools** | Read, Write, Edit, Bash, Glob, Grep |
+| **Tools** | Read, Bash, Glob, Grep |
 | **Model (balanced)** | Sonnet |
 | **Color** | Red |
-| **Produces** | `{phase}-SECURITY.md` |
+| **Produces** | Structured verdict (SECURED / OPEN_THREATS / ESCALATE) — orchestrator writes `{phase}-SECURITY.md` (#2119) |
 
 **Key behaviors:**
 - Verifies each threat by its declared disposition (mitigate / accept / transfer)