refactor: add Keymap with default bindings and wire into handle_key

Create keymap.rs with Keymap struct mapping (mode, key) to Cmd trait
objects. Wire into App::handle_key — keymap dispatch is tried first,
falling through to old handlers for unmigrated bindings. Normal mode
navigation, cell ops, mode switches, and Help mode are keymap-driven.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/command/mod.rs

@@ -7,6 +7,7 @@
 
 pub mod cmd;
 pub mod dispatch;
+pub mod keymap;
 pub mod parse;
 pub mod types;