refactor: add Default derives to CmdRegistry and Keymap

Add #[derive(Default)] to CmdRegistry and Keymap structs, enabling
easy construction of empty instances with CmdRegistry::default() and
Keymap::default().

This simplifies initialization code and follows Rust conventions for
types that hold empty collections as their default state.

Co-Authored-By: fiddlerwoaroof/git-smart-commit (unsloth/Qwen3.5-35B-A3B-GGUF:Q5_K_M)

src/command/cmd.rs

@@ -69,6 +69,7 @@ struct CmdEntry {
 }
 
 /// Registry of commands constructible from text or from interactive context.
+#[derive(Default)]
 pub struct CmdRegistry {
     entries: Vec<CmdEntry>,
 }

src/command/keymap.rs

@@ -75,6 +75,7 @@ pub enum Binding {
 }
 
 /// A keymap maps key patterns to bindings (command names or prefix sub-keymaps).
+#[derive(Default)]
 pub struct Keymap {
     bindings: HashMap<KeyPattern, Binding>,
 }