ci: auto-close inactive needs-investigation issues after 30 days

[zheli]

Summary

Adds a scheduled workflow that closes issues labeled needs investigation after 30 days with no activity. The label stays human-applied; the workflow only closes.

Changes

• Add .github/workflows/stale-needs-investigation.yml using SHA-pinned actions/stale@v11.0.0 on the self-hosted runner
• Scope to needs investigation only; ignore PRs
• Remove the label on new activity so the auto-close timer does not keep running after engagement

Test plan

• Confirm the needs investigation label name matches exactly in the repo
• Run the workflow via workflow_dispatch on this branch (or after merge on main) and check Actions logs
• Optionally label a test issue and verify close behavior after adjusting days-before-issue-close temporarily
• N/A: make test / make lint / e2e (workflow-only change)

Checklist

• Tests pass locally (N/A — workflow YAML only)
• No coverage regression on touched code (N/A)
• Docs updated (and website/ synced if any mirrored docs/*.md changed) (N/A)
• CLI ↔ Web UI parity maintained (or follow-up issue + TODO(#issue) comment) (N/A)