ci: add AI Validation workflow on circinus

Adds .github/workflows/ai-validation.yml on the circinus release branch.
Byte-identical to the version that lands on rolling via #1957.

Background
----------

For pull_request_target events, GitHub Actions evaluates the workflow
file from the repo's DEFAULT branch (rolling), so the rolling workflow
already fires on PRs targeting circinus — that's why phantom queue
entries for "AI Validation" appeared on Mergify backport PRs to
circinus even when this file didn't yet exist on the branch.

Adding the file here is therefore not a behavioral fix; it's
governance/clarity:

  * makes the workflow visible to maintainers reading the circinus
    branch in isolation,
  * preserves the workflow if rolling's copy is ever removed or
    renamed,
  * allows independent edits per release branch in the future (e.g.
    different REVIEWER_REF pin if a release branch needs a frozen
    reviewer version).

Runner pool
-----------

`runs-on: ubuntu-latest` on both prepare and validate — see #1957 for
the full rationale (the vyos org has no self-hosted runners labeled
`web`; ubuntu-latest is the only available pool for this repo).

Branches-map and reference DB
-----------------------------

The reviewer's branches.json already maps `circinus → circinus`
(see VyOS-Networks/vyos-docs-opus-reviewer/branches.json on
reviewer-v1.0.1), and the matrixed rebuild-reference workflow already
publishes `reference-db-circinus.tar.gz` as part of each rebuild. No
reviewer-side change needed.

0 files changed, 0 insertions, 0 deletions