Refresh Linear-only issue tracking policy

[yvette-carlisle]

Summary

• remove the GitHub Issues triage-label workflow now that Linear is the issue-tracking authority
• update the issue labeling guide to describe Linear labels and clarify that GitHub remains for code hosting, PR review, release, and CI
• remove stale GitHub issue-number references from current docs while preserving doc_source_ref/v1 issue_number provenance support

Verification

• git diff --check
• cargo make fmt-check
• cargo make checks

Notes

• This replaces the stale April branch contents with a current-main takeover patch.
• This PR does not change GitHub repository settings; disabling GitHub Issues, if still enabled, remains an account/repo setting action.