Add a preflight/doctor command to verify ffmpeg and optional overlay dependencies

I was going through the setup flow and noticed the README points agents at install.md to wire up ffmpeg, but there doesn't seem to be a quick way to verify the environment before starting a long edit session.

Since video-use can spawn parallel sub-agents for overlays and later self-evaluates rendered output, a missing ffprobe, unsupported ffmpeg build, or broken Remotion/Manim install might not surface until a render step deep in the process. That can make failures harder to diagnose and wastes time on partial renders.

A small preflight command would help a lot, something like python scripts/check_env.py or make doctor. It could print ok/fail for ffmpeg/ffprobe, confirm the footage/output directories are writable, and optionally check overlay backends only if they are configured. If it returned a non-zero exit code on failure, agents could run it automatically after install and stop before doing expensive work.

This would also give the setup prompt a concrete verification step after install.md, and it would help users across macOS/Linux/WSL where ffmpeg installs can vary.