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)

Cargo.toml

@@ -23,6 +23,7 @@ unicode-width = "0.2"
 dirs = "5"
 csv = "1"
 clap = { version = "4.6.0", features = ["derive"] }
+enum_dispatch = "0.3.13"
 
 [dev-dependencies]
 proptest = "1"