Add multi-platform git credential management

- Add git-credential-oauth package for OAuth authentication
- Configure OAuth helpers for GitHub, GitLab, BitBucket
- Add token-based authentication for Overleaf
- Remove disabled credential helper configuration
- Support secure browser-based OAuth flows and token storage

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

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

modules/git.nix

@@ -1,6 +1,10 @@
 { config, pkgs, ... }:
 
 {
+  # Enable git-credential-oauth for GitHub, GitLab, BitBucket
+  programs.git-credential-oauth = {
+    enable = true;
+  };
   programs.git = {
     enable = true;
     
@@ -79,7 +83,17 @@
     ];
     
     extraConfig = {
-      credential.helper = "";
+      # Platform-specific credential configuration
+      credential = {
+        # OAuth platforms (handled by git-credential-oauth)
+        "https://github.com".helper = "oauth";
+        "https://gitlab.com".helper = "oauth";
+        "https://bitbucket.org".helper = "oauth";
+        
+        # Token-based platforms
+        "https://git.overleaf.com".helper = "store";
+        "https://git.overleaf.com".username = "git";
+      };
       
       core = {
         editor = "nvim";