Reorganize configuration into modular directory structure

- Create modules/ directory for Nix configuration modules
- Create config/ directory for configuration files
- Move nvim.nix, tmux.nix, zsh.nix to modules/
- Move p10k.zsh to config/
- Update flake.nix import paths to reference new structure
- Update zsh.nix p10k.zsh path reference
- Update README.md architecture documentation
- Keep tmux.sh at root for easy accessibility

This provides better separation between Nix modules and config files
while maintaining a clean, scalable structure.

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

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

README.md

@@ -27,11 +27,13 @@ home-manager switch --flake github:Logan-Lin/nix-config#yanlin
 ```
 .
 ├── flake.nix          # Main flake configuration and package definitions
-├── nvim.nix           # Neovim configuration with plugins and keymaps
-├── tmux.nix           # Tmux setup with vim-like navigation
-├── zsh.nix            # Zsh with Powerlevel10k and modern CLI tools
-├── p10k.zsh           # Powerlevel10k theme configuration (managed by Nix)
-└── tmux.sh            # Tmux session automation script
+├── tmux.sh            # Tmux session automation script
+├── modules/           # Nix configuration modules
+│   ├── nvim.nix       # Neovim configuration with plugins and keymaps
+│   ├── tmux.nix       # Tmux setup with vim-like navigation
+│   └── zsh.nix        # Zsh with Powerlevel10k and modern CLI tools
+└── config/            # Configuration files
+    └── p10k.zsh       # Powerlevel10k theme configuration
 ```
 
 ## 🛠️ Software Configurations