[SE-3-06] Page-local MDX safety check

[DeveloperAlly]

Tracking issue for SE-3-06 in the Sprint 3 + Post-Migration Audit stream.

Scope:

• Implement Page-local MDX safety check
• Keep changes isolated to the task branch/worktree
• Validate against docs-v2 governance rules before PR

[DeveloperAlly]

Validated on docs-v2 on March 11, 2026. Closing as implemented by tools/scripts/validators/content/check-mdx-safe-markdown.js, exposed through the test:mdx:safe script in tools/package.json.