Improve triage throughput and fix workflow bugs (#15769)

- Increase issue-repro-triage schedule from daily to every 12h
- Process up to 3 backlog issues per run (was 1)
- Add explicit step to triage external contributor issues on schedule
- Bump safe-output limits (comments 3→10, labels 5→15, PRs 2→3)
- Bump timeout 30→45 min for increased workload
- Add PR description validation to pr-iteration
- Fix duplicate reviewer footer in pr-expert-reviewer
- Add @nohwnd mention and label counting to daily-qa

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>