Add doc lint check to CI lint-gate

The precheck script runs lint_docs.py (step 5/5) but CI's lint-gate
job was missing it. This meant Doxygen % escaping issues and other doc
reference errors were not caught in CI.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: sbryngelson

.github/workflows/test.yml

@@ -49,6 +49,9 @@ jobs:
         run: |
           ! grep -iR -e '\.\.\.' -e '\-\-\-' -e '===' ./src/*
 
+      - name: Lint Docs
+        run: python3 toolchain/mfc/lint_docs.py
+
   file-changes:
     name: Detect File Changes
     runs-on: 'ubuntu-latest'