fix(api): Zod-validate POST /api/github-skills + document new gate envs + pin merge-integrity actions

Three latent heavy-CI reds surfaced by the VPS validation dispatch (the fast
path never runs these gates):

- t06 route-validation: POST /api/github-skills destructured request.json()
  blind — a non-array 'targets' would .map-crash. Now validateBody(zod)
  with defaults preserved (Hard Rule #7). Guard:
  tests/unit/github-skills-route-validation.test.ts (4/4).
- env-doc-sync: document OMNIROUTE_SKIP_SYSTEM_TRUST (#6310) and the
  changelog-integrity gate envs CHANGELOG_BASE_REF/ALLOW_CHANGELOG_REMOVALS
  (#6300) in .env.example + ENVIRONMENT.md.
- zizmor ratchet: the new merge-integrity job's checkout/setup-node uses were
  unpinned (+1 finding, 160 > baseline 159); pinned by SHA -> 158 (< baseline).

.github/workflows/quality.yml

@@ -195,11 +195,11 @@ jobs:
       API_KEY_SECRET: ci-lint-api-key-secret-long
       DISABLE_SQLITE_AUTO_BACKUP: "true"
     steps:
-      - uses: actions/checkout@v7
+      - uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7
         with:
           fetch-depth: 0
           persist-credentials: false
-      - uses: actions/setup-node@v6
+      - uses: actions/setup-node@48b55a011bda9f5d6aeb4c2d9c7362e8dae4041e # v6
         with:
           node-version: ${{ env.CI_NODE_VERSION }}
           cache: npm