ci: add AI Validation workflow on sagitta

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

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 sagitta — that's why phantom queue
entries for "AI Validation" appeared on Mergify backport PRs to
sagitta (#1955) 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 sagitta
    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 `sagitta → sagitta`
(see VyOS-Networks/vyos-docs-opus-reviewer/branches.json on
reviewer-v1.0.1), and the matrixed rebuild-reference workflow already
publishes `reference-db-sagitta.tar.gz` as part of each rebuild. No
reviewer-side change needed.

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