fix(pr-sous-chef): prefilter step summary + per-run report issue (#43494)

* fix(pr-sous-chef): add prefilter step summary and create-issue run report

- Add GITHUB_STEP_SUMMARY output in fetch-prs step showing candidate/filter/eligible counts
- Add create-issue safe-output with close-older-issues:true and 3d expiry
- Update token efficiency rules: use create_issue for run summary instead of noop
- Update Run summary section: require create_issue with explicit no-agent-assignment header

Co-authored-by: pelikhan <4175913+pelikhan@users.noreply.github.com>

* fix(pr-sous-chef): clarify fallback noop message format in run summary

Co-authored-by: pelikhan <4175913+pelikhan@users.noreply.github.com>

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Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: pelikhan <4175913+pelikhan@users.noreply.github.com>

Author: Copilot