Implement CI Gatekeeper Workflows for Triaged Issues

Background:
Recently, open-source repositories have been dealing with an influx of drive-by AI-generated PRs. This adds noise and places unnecessary strain on maintainers and CI resources. To manage this and save CI compute time, we need to implement a gatekeeping mechanism similar to the one introduced in [Kubeflow Pipelines PR #13293](https://www.google.com/search?q=https://github.com/kubeflow/pipelines/pull/13293).

Proposed Solution:
Implement a two-part GitHub Actions workflow to ensure that heavy CI workflows only trigger for PRs that are linked to an approved/triaged issue.

Issue Triage Workflow (issue-triage.yml):

Automatically responds to newly opened issues with a welcome message (e.g., "Thanks for opening this issue! A maintainer will triage it shortly.").

Optionally assigns a default label such as needs-triage.

CI Gatekeeper Workflow (ci-gatekeeper.yml):

Acts as a blocker for main CI actions.

On PR creation or modification, the workflow should parse the PR description to find the linked issue.

Verify that the linked issue has a specific label (e.g., ready or triaged) indicating maintainer approval.

If the issue is not triaged, block the CI from running and leave a comment explaining that the PR requires an approved issue before CI checks can proceed.

Acceptance Criteria:

[ ] Create .github/workflows/issue-triage.yml to handle automatic welcome messaging on new issues.

[ ] Create .github/workflows/ci-gatekeeper.yml to validate that a PR references an open issue.

[ ] Ensure the CI gatekeeper correctly checks for a ready or equivalent triage label on the referenced issue.

[ ] Update our contribution guidelines/README to document this new process so contributors know they need an approved issue before submitting a PR.

[ ] Verify that downstream CI jobs only trigger once the gatekeeper job passes

Start with the acceptance criteria and inspect the repository's existing CI entry points before creating .github/workflows/issue-triage.yml and .github/workflows/ci-gatekeeper.yml. Update the contribution guidelines or README, then verify that linked issues, triage labels, comments, and downstream CI triggering behave as specified.

Written by the indexing model from the issue text.