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>

modules/git.nix

@@ -0,0 +1,72 @@
+{ config, pkgs, ... }:
+
+{
+  programs.git = {
+    enable = true;
+    
+    userName = "Yan Lin";
+    userEmail = "github@yanlincs.com";
+    
+    extraConfig = {
+      credential.helper = "";
+      
+      core = {
+        editor = "nvim";
+        autocrlf = "input";
+        ignorecase = false;
+      };
+      
+      init.defaultBranch = "main";
+      
+      push = {
+        default = "simple";
+        autoSetupRemote = true;
+      };
+      
+      pull = {
+        rebase = true;
+      };
+      
+      merge = {
+        conflictstyle = "diff3";
+      };
+      
+      diff = {
+        colorMoved = "default";
+      };
+      
+      status = {
+        showUntrackedFiles = "all";
+      };
+    };
+    
+    aliases = {
+      st = "status";
+      co = "checkout";
+      br = "branch";
+      ci = "commit";
+      unstage = "reset HEAD --";
+      last = "log -1 HEAD";
+      visual = "!gitk";
+      
+      # Better logging
+      lg = "log --color --graph --pretty=format:'%Cred%h%Creset -%C(yellow)%d%Creset %s %Cgreen(%cr) %C(bold blue)<%an>%Creset' --abbrev-commit";
+      lga = "log --color --graph --pretty=format:'%Cred%h%Creset -%C(yellow)%d%Creset %s %Cgreen(%cr) %C(bold blue)<%an>%Creset' --abbrev-commit --all";
+      
+      # Quick operations
+      up = "pull --rebase";
+      cm = "commit -m";
+      ca = "commit --amend";
+      
+      # Show changes
+      d = "diff";
+      dc = "diff --cached";
+      ds = "diff --stat";
+      
+      # Stash operations
+      sl = "stash list";
+      sp = "stash pop";
+      ss = "stash save";
+    };
+  };
+}