Git Commit Video Walkthrough Generator

Build the code walkthrough app as is described in IMPLEMENTATION_PLAN.md. Completed when: - The user can call the CLI tool in a Git repository and specify either a Git commit or branch name, whereupon a video is created that walks through the code that is introduced in this commit or branch. - The video shows the code while a voice is giving a summary of what the code changes do. - Output: <promise>DONE</promise>