ci(ai-validation): switch to GitHub-hosted ubuntu-latest runners

The `vyos` org does not have self-hosted runners labeled `web` (those
live in the VyOS-Networks org pool and only serve repos there). Every
AI Validation run queued since #1947 merged sat in `queued` state
indefinitely with no runner picking it up — observed across all recent
PRs (#1955 mergify backports, #1956, plus several yuriy/* branches).

Switching both `prepare` and `validate` jobs to `runs-on: ubuntu-latest`:

* Removes the host-isolation half of the prepare-job rationale comment
  and replaces it with the ephemeral-VM rationale (cross-run state
  leakage is impossible on a fresh GitHub-hosted VM).
* Removes both `atos-actions/clean-self-hosted-runner` cleanup steps —
  GitHub-hosted runners are ephemeral, the action is a no-op there at
  best and a failure mode at worst (it expects self-hosted workspace
  patterns that don't exist on hosted runners).
* Tweaks one comment that mentioned `/proc/<pid>/cmdline on the self-
  hosted runner` to be runner-agnostic.

Also removes `.github/actionlint.yaml`. It was added in #1947 to silence
actionlint's "label 'web' is unknown" false positive — with no workflow
in this repo now using `[self-hosted, web]`, the file is dead code.

The canonical reference at `VyOS-Networks/vyos-docs-opus-reviewer/scripts/
ai-validation.yml` intentionally diverges: that repo IS in VyOS-Networks
and has access to the `web` self-hosted pool, so its canonical keeps
`runs-on: [self-hosted, web]` and the cleanup steps. The deployed
file's REFERENCE COPY header comment block in the reviewer repo will be
updated in a follow-up to note that the deployed file may use different
runners per host repo's pool availability.

No security regression — the trust boundary on prepare is enforced by
no-fork-code-execution, no-secrets-referenced, persist-credentials:false,
and the split-job artifact, all of which are unchanged. Adds the implicit
host-ephemerality guarantee of GitHub-hosted runners.

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