Replace gitui with lazygit for better git TUI experience

- Replace gitui package with lazygit in flake.nix
- Update all project templates to launch lazygit instead of gitui
- Update README documentation with lazygit features and capabilities
- Maintain same tmux workflow and window layouts

Benefits of lazygit:
- More intuitive and user-friendly interface
- Better performance and responsiveness
- Enhanced file tree navigation with git status
- Comprehensive keyboard shortcuts and help system
- Active development with regular feature updates

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

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

flake.nix

@@ -66,7 +66,7 @@
         claude-code.packages.aarch64-darwin.claude-code
         nerd-fonts.fira-code
         nerd-fonts.jetbrains-mono
-        gitui
+        lazygit
         ncdu
         git-credential-oauth
       ];