refactor(command): decouple keymap bindings from command implementations

Refactor the keymap system to use string-based command names instead of
concrete command struct instantiations. This introduces a Binding enum that
can represent either a command lookup (name + args) or a prefix sub-keymap.

Key changes:
- Keymap now stores Binding enum instead of Arc<dyn Cmd>
- dispatch() accepts CmdRegistry to resolve commands at runtime
- Added bind_args() for commands with arguments
- KeymapSet now owns the command registry
- Removed PrefixKey struct, inlined its logic
- Updated all default keymap bindings to use string names

This enables more flexible command configuration and easier testing.

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

src/ui/app.rs

@@ -141,7 +141,8 @@ impl App {
         if let Some(transient) = self.transient_keymap.take() {
             let effects = {
                 let ctx = self.cmd_context(key.code, key.modifiers);
-                transient.dispatch(&ctx, key.code, key.modifiers)
+                self.keymap_set
+                    .dispatch_transient(&transient, &ctx, key.code, key.modifiers)
             };
             if let Some(effects) = effects {
                 self.apply_effects(effects);