fix(#1864): balance <step> tags in settings-advanced.md §8 Model Policy (#2014)

* fix(#1864): balance <step> tags in settings-advanced.md §8 Model Policy

§8 Model Policy ended with </step> but had no matching opening tag (5 opens /
6 closes), leaving it as loose inter-step content. Add the missing
<step name="model_policy"> opener so the section is a proper step.

- gsd-core/workflows/settings-advanced.md: add <step name="model_policy">
- tests/workflow-step-tag-balance.test.cjs: regression guard — every top-level
  workflow must have balanced <step>/</step> (fenced code stripped), plus a
  focused assertion that §8 is wrapped in model_policy.
- goldens + workflow-size baseline recaptured.

Closes #1864

* docs(#1864): backfill changeset pr 2014

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

@@ -352,7 +352,7 @@
   "gsd-core/workflows/scan.md": "003883d71c37da7d",
   "gsd-core/workflows/secure-phase.md": "29fc6b62c5c5dc62",
   "gsd-core/workflows/session-report.md": "2e5b1205324ddefa",
-  "gsd-core/workflows/settings-advanced.md": "3d86a02a58171cff",
+  "gsd-core/workflows/settings-advanced.md": "b57c947edee905bc",
   "gsd-core/workflows/settings-integrations.md": "70515c5838fb9826",
   "gsd-core/workflows/settings.md": "b2d39d972650f5f3",
   "gsd-core/workflows/ship.md": "7b8fe9f89143e648",