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.lock

@@ -339,6 +339,18 @@ version = "1.15.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "48c757948c5ede0e46177b7add2e67155f70e33c07fea8284df6576da70b3719"
 
+[[package]]
+name = "enum_dispatch"
+version = "0.3.13"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "aa18ce2bc66555b3218614519ac839ddb759a7d6720732f979ef8d13be147ecd"
+dependencies = [
+ "once_cell",
+ "proc-macro2",
+ "quote",
+ "syn",
+]
+
 [[package]]
 name = "equivalent"
 version = "1.0.2"
@@ -494,6 +506,7 @@ dependencies = [
  "crossterm",
  "csv",
  "dirs",
+ "enum_dispatch",
  "flate2",
  "indexmap",
  "proptest",