[CI Broken] BuildKite CI not running or not reporting status

Summary

BuildKite CI appears to be either not running or not properly integrated with GitHub's status check API.

Evidence

1. No Status Checks Visible

After creating PR #46 (CI Health Check) and waiting over 2 hours:

• GitHub API returns "statusCheckRollup": null for all PRs, including the health check PR
• gh pr checks 46 reports: "no checks reported on the 'main' branch"
• No CI status indicators appear on any PRs (checked PRs #46, #45, #44)

2. BuildKite Configuration Exists

The repository has BuildKite configuration in .buildkite/pipeline.yml with:

• Format check step
• CPU test matrix (Core, Applications, QA)
• CUDA test matrix (Core, Applications)
• All configured to run unless [skip tests] is in commit message

3. Recent PR Merge Patterns

• PR #45 (merged 2025-12-29): created at 21:33:43Z, merged at 21:54:22Z (21 minutes)
• This is suspiciously fast if full CI test suite was required to pass

4. BuildKite Integration History

• BuildKite was added in PR #30 (merged 2023-07-21)
• Configuration has been in place for over a year

Root Cause Analysis

Possible causes:

1. BuildKite webhook not configured: GitHub may not be triggering BuildKite builds on PR creation/updates
2. BuildKite status integration disabled: BuildKite builds may run but not report status back to GitHub
3. BuildKite pipeline disabled: The BuildKite pipeline may be paused or disabled in BuildKite's dashboard
4. Authentication/permissions issue: BuildKite may lack permissions to update GitHub PR status
5. Fork PR restrictions: BuildKite may not be configured to run on PRs from forks (like PR #46 from ChrisRackauckas-Claude fork)

Impact

• No automated testing verification before merging PRs
• Potential for broken code to be merged into main branch
• Loss of CI safety net for quality assurance

Suggested Fix Approach

1. Verify BuildKite pipeline status:

   • Check BuildKite dashboard to see if pipeline exists and is enabled
   • Verify webhook configuration between GitHub and BuildKite

2. Check GitHub integration:

   • Ensure BuildKite GitHub app is installed and has proper permissions
   • Verify BuildKite is configured to report commit statuses

3. Test with direct push:

   • Try pushing directly to a branch (not from fork) to rule out fork-related issues

4. Review BuildKite settings:

   • Check if there are restrictions on which branches/PRs trigger builds
   • Verify that the BuildKite organization/pipeline is still active

Additional Context

• Health check PR: #46
• CI configuration: .buildkite/pipeline.yml
• Last known working CI: Unknown (no recent evidence of CI actually running)

cc @ChrisRackauckas

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Discovered during automated CI health check on 2025-12-30

Start with .buildkite/pipeline.yml and inspect PR #46 with gh pr checks 46; then verify the BuildKite pipeline, webhook, GitHub app permissions, and fork restrictions in the dashboard. Done means a test build is triggered for a pull request and its result appears as a GitHub status check.