ci(ai-validation): skip prepare on Mergify-authored PRs

Lifts the existing Mergify-author short-circuit (today inside validate's
`secrets-check` step) to a job-level `if:` on `prepare`, so the whole
pipeline skips for backport/queue PRs.

Why now: every Mergify backport whose merge ref shares no shallow
ancestor with the (advanced) base branch fails the prepare step at

  git diff "$BASE...HEAD" --name-only ...
  fatal: FETCH_HEAD...HEAD: no merge base

(because base is `git fetch --no-tags --depth=1` and the merge ref is
`fetch-depth: 2`). Proximate symptom: run 25842928620 on PR #2042
(sagitta backport of #2023). AI Validation isn't a required check so
the queue isn't blocked, but every Mergify backport is left with a red
"prepare" check that adds noise to PR review.

The validate-level skip in commit 0e8a2956 was correct for the
"claude-code-action rejects bot-initiated runs" failure mode but
fires too late — prepare has already run and crashed before validate's
`if: needs.prepare.outputs.has_md_changes == 'true'` even evaluates.

Implementation: single job-level `if:` on prepare. validate's
`needs: [prepare]` cascades the skip naturally (skipped needs make
the dependent's expression-based `if:` evaluate against empty outputs).
The in-step author check in validate stays as defense-in-depth.

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

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