feat: use enum_dispatch for command dispatch and add Open command

Replace manual dynamic dispatch using Box <dyn Runnable> with enum_dispatch
for improved performance and cleaner code.

Add a new Open command to allow opening the TUI as the default behavior
when no subcommand is provided.

Co-Authored-By: fiddlerwoaroof/git-smart-commit (unsloth/gemma-4-26B-A4B-it-GGUF:UD-Q5_K_XL)

flake.nix

@@ -42,6 +42,7 @@
           pkgs.pkg-config
           pkgs.rust-analyzer
           crate2nix.packages.${system}.default
+          pkgs.cargo-expand
         ];
         RUST_BACKTRACE = "1";
       };