fix(triage-issue): Fix Linear idempotency check to match actual report header

[github-actions]

Note

The pull request "fix(triage-issue): Fix Linear idempotency check to match actual report header" was created by @chargome but did not reference an issue. Therefore this issue was created for better visibility in external tools like Linear.

The duplicate-comment guard in post_linear_comment.py was checking for comments starting with ## Automated Triage Report, but the triage report template (triage-report.md) generates headers in the format ## Issue Triage: #<number>.

[linear]

JS-1792