feat(keymap): expose registry and update keybinding

Expose CmdRegistry via KeymapSet registry method.
Update Z keybinding to use SaveAndQuit command.
Pass registry to App's CmdContext construction.

Co-Authored-By: fiddlerwoaroof/git-smart-commit (bartowski/nvidia_Nemotron-Cascade-2-30B-A3B-GGUF)

src/ui/app.rs

@@ -160,6 +160,7 @@ impl App {
         CmdContext {
             model: &self.model,
             layout,
+            registry: self.keymap_set.registry(),
             mode: &self.mode,
             selected: view.selected,
             row_offset: view.row_offset,