feat: add automated triage workflow for community issues

[stack72]

• Adds a GitHub Actions workflow that fires when an issue is opened
• Checks author_association from the webhook payload to determine if the author is an OWNER, MEMBER, or
  COLLABORATOR
• For external contributors: adds a needs-triage label and posts a welcome comment thanking them and asking
  for reproduction details
• Maintainers/owners/collaborators opening issues are unaffected

[github-actions]

✅ Approved

This PR adds a well-structured GitHub Actions workflow for automated community issue triage.

Review Summary

No blocking issues found.

What Was Reviewed

• Workflow Logic: Correctly checks author_association from the webhook payload to identify external contributors
• Security: Properly scoped permissions (issues: write), uses secrets.GITHUB_TOKEN, no injection vulnerabilities
• CLAUDE.md Compliance: N/A - this is a YAML workflow file, not TypeScript code requiring license headers, tests, etc.
• DDD Principles: N/A - infrastructure automation, not domain code

Minor Suggestions (Non-blocking)

1. Consider documenting the workflow in the repo's contributing guidelines so maintainers know issues from external contributors will get the needs-triage label automatically
2. The needs-triage label will be auto-created if it doesn't exist, but you may want to pre-create it with a description and color for consistency

The implementation is clean, follows GitHub Actions best practices, and serves its intended purpose well.