bug(yt-dlp): link parsed as an option can re-enable plugin execution #452

Security review finding (medium, CWE-88).

downloadVideo in packages/yt-dlp/src/download.ts appends the URL as a bare positional argument with no -- separator, and the exported function trusts its caller to have run isVideoUrl. A value shaped like --plugin-dirs=<dir> is parsed by yt-dlp as an option: it re-enables the plugin search --no-plugin-dirs had just disabled, loading and running Python code from a caller-named directory. Other option-shaped values (--exec, --paths, --config) are injectable the same way.

Proposed fix: assert http/https inside downloadVideo and pass the link behind a -- end-of-options separator.

Fix: #444