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/opencode.json

@@ -352,7 +352,7 @@
   "gsd-core/workflows/scan.md": "ad8ebcad4626d4a8",
   "gsd-core/workflows/secure-phase.md": "e9a488cec3b4efdc",
   "gsd-core/workflows/session-report.md": "2e5b1205324ddefa",
-  "gsd-core/workflows/settings-advanced.md": "72438bf453f7340b",
+  "gsd-core/workflows/settings-advanced.md": "f62c8e4fa6d3a3d9",
   "gsd-core/workflows/settings-integrations.md": "d1711a95f44fdbde",
   "gsd-core/workflows/settings.md": "bbbcad8d95d5e813",
   "gsd-core/workflows/ship.md": "924c79e3cfd1e42b",