Add SSH configuration management via nix

- Create modules/ssh.nix with all host configurations
- Import SSH module in flake.nix homeConfiguration
- Manage SSH config declaratively through home-manager

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

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

flake.nix

@@ -43,6 +43,7 @@
         ./modules/nvim.nix 
         ./modules/tmux.nix 
         ./modules/zsh.nix 
+        ./modules/ssh.nix
       ];
 
       home.username = "yanlin";