Add ci-notify-ai-conformance composite action

[pascalbreuninger]

Summary

• New YAML-only composite action for Slack notifications from the K8s AI Conformance pipeline
• Follows the same pattern as ci-notify-nightly-tests and release-notification
• Inputs: status, k8s_version, vcluster_version, tenancy_model, artifact_url, webhook_url

Context

The AI conformance pipeline (loft-sh/vcluster-pro#1630) currently has inline Slack notification steps. Per review feedback, these should use a reusable composite action from this repo.

Once merged, the vcluster-pro workflow will be updated to call:

- uses: loft-sh/github-actions/.github/actions/ci-notify-ai-conformance@<sha>
  with:
    status: ${{ steps.status.outputs.status_text }}
    k8s_version: ${{ inputs.k8s_version }}
    vcluster_version: ${{ inputs.vcluster_version }}
    tenancy_model: ${{ inputs.tenancy_model }}
    webhook_url: ${{ secrets.SLACK_WEBHOOK_URL_CI_TESTS_ALERTS }}

Security checklist

• No persist-credentials needed (no checkout step)
• No explicit permissions needed (composite action, not workflow)
• Slack action pinned by SHA (af78098f536edbc4de71162a307590698245be95 = v3.0.1)
• YAML-only composite — no unit tests needed per conventions
• Fork PRs: N/A (action is called by workflows, not triggered by PRs)

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