ci: require nvskills status for skill changes

[yashrajp22]

Summary

• Add a reusable workflow guard that fails when watched skill paths changed but the current PR head SHA does not have a successful NVSkills CI status.
• The guard covers skills/, team-skills/, rules/team-rules/, and plugins/.
• Keeps the existing /nvskills-ci dispatch flow unchanged.

Validation

• Parsed .github/workflows/team-request.yml as YAML
• Ran bash -n on the inline guard script
• Ran git diff --check

[mosheabr]

Thanks @yashrajp22 — this closes a real gap and the overall shape is right. A few things before it can land:

Blockers

1. DCO is failing — please git rebase --signoff origin/main && git push --force-with-lease.
2. The guard needs to exempt the automated/sync-skills bot PRs (same pattern as the DCO/authors checks). Sync PRs touch skills/** with content signed in the source repos, so their head SHAs never carry an NVSkills CI status — as written this fails every daily sync PR.

Design

3. The status: trigger won't clear a red check on the PR: status-event runs execute against the default branch, so the new check run doesn't attach to the PR head SHA. Authors would need a manual re-run after /nvskills-ci succeeds. Consider having the NVSkills CI pipeline refresh the check itself, or a workflow_run-based re-check.
4. Please drop the paths: filter on the pull_request trigger. Once this check is required in branch protection, PRs that don't touch watched paths (e.g. components.d/ onboarding PRs) would never report the check and sit at "Expected" forever. The job already self-skips via the files API, so it's safe to run on every PR.

Minor

5. The push branch of the guard's if is unreachable (the caller only forwards signature pushes, which the guard excludes) — can be removed.
6. The failure message should note that /nvskills-ci only works on branches in NVIDIA/skills, not forks, so fork authors know to move their branch rather than re-commenting.

Happy to re-review once updated.