ci(ai-validation): skip Pass 1/2 when PR has no .md changes

actions/upload-artifact silently omits empty directories. On a PR that
changes no docs/**/*.md (workflow tweaks, README edits, config), the
prepare job's _changed_md/ ends up empty and is dropped from the
artifact. The validate job downloads the artifact, then "Pass 1 —
deterministic checks" tries to start with `working-directory:
_changed_md`, which doesn't exist, and bash fails before any in-step
short-circuit can run.

Surface a `has_md_changes` output from prepare based on whether
changed-md.txt is non-empty, and gate Pass 1 + Pass 2 on it. When the
flag is false, both review steps skip cleanly with no failure noise.

Affects all infrastructure-only PRs (CI changes, workflow updates,
config tweaks). Same failure was visible on the in-flight backports
of update-version-tags hardening (#1965, #1966) which are pure
workflow changes.

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

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