Before: missing node → hard exit; missing browser → FileNotFoundError.
After: both try ensure_dependency() first, which prompts interactively
and delegates installation to install.sh --ensure.
ripgrep and ffmpeg already degrade gracefully (grep fallback, skip
conversion) so they don't need wiring.
Also documents the design rationale in dep_ensure.py: detection and
prompting live in Python (portable, instant, UX-integrated); only
the actual installation delegates to install.sh (1900 lines of
battle-tested OS/package-manager logic).