ci(ai-validation): cover deletion-only Markdown PRs

CodeRabbit caught a real Major issue on this PR's first version:
has_md_changes was derived from changed-md.txt, which is built with
--diff-filter=ACMRT (excludes Deletes). A PR that only deletes
docs/**/*.md files would set has_md_changes=false and skip validate
entirely — but Pass 1 reviews the diff (via --pr-diff
../diff-md.patch), not just the post-image files in _changed_md/, so
deletes are legitimate review targets.

Two fixes that go together:

1. Derive has_md_changes from diff-md.patch (the unfiltered git diff
   already computed) instead of changed-md.txt. diff-md.patch includes
   D entries; changed-md.txt does not.

2. Add a defensive `mkdir -p _changed_md` after artifact download.
   actions/upload-artifact silently omits empty directories — on a
   deletion-only PR the artifact carries no _changed_md/ at all, and
   Pass 1's `working-directory: _changed_md` would fail. The
   directory is recreated empty; Pass 1 still operates on the diff.

🤖 Generated by [robots](https://vyos.io)

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