search_files(target='files') now uses rg --files -g instead of find.
Ripgrep respects .gitignore, excludes hidden dirs by default, and has
parallel directory traversal — ~200x faster on wide trees (0.14s vs 34s
benchmarked on 164-repo tree).
Falls back to find when rg is unavailable, preserving hidden-dir
exclusion and BSD find compatibility.
Salvaged from PR #1464 by @light-merlin-dark (Merlin) — adapted to
preserve hidden-dir exclusion added since the original PR.