fix(ci): fail-fast on non-regular docs entries in PR diff

Copilot finding on .github/workflows/ai-validation.yml:86:

  The bundling loop currently *skips* non-regular tree entries
  (symlinks mode 120000, submodules 160000) with a `::warning::` and
  continues. A PR that converts a regular docs/**/*.md into a symlink
  effectively bypasses both Pass 1 (no file copied into _changed_md/)
  and Pass 2 (LLM Read/Glob/Grep sees no content) — reducing
  validation coverage on exactly the kind of change that warrants
  closer review.

Replaced the `continue` with `exit 1` on non-regular mode. The error
message instructs the PR author to convert the file back to a regular
.md (or get explicit maintainer waiver). Maintainers can still land
non-regular doc entries by adjusting the workflow, but the decision
becomes visible rather than silent.

Combined with the symlink-exfil mitigation (commit 1ea164ff): we no
longer copy symlink target content into the artifact AND we no longer
silently skip the change. Either it's a regular file we can validate,
or the workflow fails loudly.

Same change being applied to the canonical reference copy in
VyOS-Networks/vyos-docs-opus-reviewer PR #13.

Suppressed-by-Copilot finding (line 110, id-token: write):
Same pushback as previous rounds. claude-code-action@v1 uses OIDC
internally (verified via this repo's commit b18a399c). Keeping.

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

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