Expand /cherry-pick workflow eligibility to project committers

[Copilot]

The /cherry-pick workflow was too restrictive: it allowed only PR author/reviewer/merger roles, which blocked some repository committers from triggering cherry-picks. This updates authorization to align with maintainer workflow needs while keeping PR-author access intact.

• Permission model update (.github/workflows/cherry-pick.yml)

  • Replaced reviewer/merger-based checks with:
    • PR author allowlist
    • repository collaborator permission check (write/maintain/admin)
  • Added clearer handling for collaborator permission lookup failures and explicit fallback behavior for non-eligible users.

• User-facing behavior/docs alignment (docs/pages/userGuide.md)

  • Updated /cherry-pick command documentation to reflect the new eligibility model: PR author or project committer.

• Authorization logic (example)

  case "$PERMISSION" in
    admin|maintain|write)
      echo "✅ Commenter is a project committer with $PERMISSION permission"
      exit 0
      ;;
    *)
      ;;
  esac