feat: add git init to post-generation hook

[h0pers]

Summary

• Add init_git() to the cookiecutter post-generation hook
• Initializes a git repo, stages all files, and creates an initial commit after project generation
• Gracefully warns if git is unavailable instead of crashing

Test plan

• Generate a new project and verify a git repo is initialized with an initial commit
• Generate a project without git installed and verify the warning message appears