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/secure-phase-single-writer.test.cjs

@@ -0,0 +1,73 @@
+/**
+ * Regression test for #2119: /gsd-secure-phase dual SECURITY.md writers.
+ *
+ * The gsd-security-auditor agent must NOT have Write/Edit tools — the
+ * orchestrator (secure-phase.md Step 6) is the sole SECURITY.md writer.
+ * The auditor returns a structured verdict; it never writes files.
+ */
+
+const { describe, test } = require('node:test');
+const assert = require('node:assert/strict');
+const fs = require('fs');
+const path = require('path');
+
+const AGENT_PATH = path.join(__dirname, '..', 'agents', 'gsd-security-auditor.md');
+
+function parseYamlTools(content) {
+  const lines = content.split(/\r?\n/);
+  let inFrontmatter = false;
+  let inTools = false;
+  const tools = [];
+  for (const line of lines) {
+    const trimmed = line.trim();
+    if (trimmed === '---') {
+      inFrontmatter = !inFrontmatter;
+      if (!inFrontmatter) break;
+      continue;
+    }
+    if (!inFrontmatter) continue;
+    if (trimmed.startsWith('tools:')) {
+      inTools = true;
+      continue;
+    }
+    if (inTools) {
+      if (trimmed.startsWith('- ')) {
+        tools.push(trimmed.slice(2).trim());
+      } else if (trimmed && !trimmed.startsWith('#')) {
+        inTools = false;
+      }
+    }
+  }
+  return tools;
+}
+
+describe('#2119 — security auditor is return-only (no file writes)', () => {
+  const content = fs.readFileSync(AGENT_PATH, 'utf-8');
+
+  test('auditor tools do not include Write or Edit', () => {
+    const tools = parseYamlTools(content);
+    assert.ok(tools.length > 0, 'tools list should be non-empty');
+    assert.ok(
+      !tools.includes('Write'),
+      `Write must not be in auditor tools (got: ${tools.join(', ')}) — orchestrator is the sole SECURITY.md writer (#2119)`,
+    );
+    assert.ok(
+      !tools.includes('Edit'),
+      `Edit must not be in auditor tools (got: ${tools.join(', ')}) — orchestrator is the sole SECURITY.md writer (#2119)`,
+    );
+  });
+
+  test('auditor description does not claim to produce SECURITY.md', () => {
+    const lines = content.split(/\r?\n/);
+    const descLine = lines.find((l) => l.startsWith('description:'));
+    assert.ok(descLine, 'description field must exist');
+    assert.ok(
+      !descLine.includes('Produces SECURITY.md'),
+      'description must not claim to produce SECURITY.md — auditor returns a verdict, orchestrator writes (#2119)',
+    );
+    assert.ok(
+      descLine.includes('Returns structured') || descLine.includes('returns'),
+      'description should state the auditor returns a structured verdict',
+    );
+  });
+});