Add Git configuration management via nix

- Create modules/git.nix with user info and useful settings
- Configure git to use SSH (disable credential helper)
- Add comprehensive git aliases and sensible defaults
- Import git module in flake.nix homeConfiguration

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

flake.nix

@@ -44,6 +44,7 @@
         ./modules/tmux.nix 
         ./modules/zsh.nix 
         ./modules/ssh.nix
+        ./modules/git.nix
       ];
 
       home.username = "yanlin";