feat(tui,coding-agent): add shell-style keybindings alt+b, alt+f, ctrl+d

- Add alt+b/alt+f as alternative bindings for cursorWordLeft/cursorWordRight
- Add ctrl+d as alternative binding for deleteCharForward
- Fix ctrl+d in custom editor to perform delete when text is present
  (previously it was always consumed, even with non-empty input)

Closes #1043

Co-authored-by: Jason Ish <ish@unx.ca>

packages/coding-agent/src/modes/interactive/components/custom-editor.ts

@@ -61,8 +61,9 @@ export class CustomEditor extends Editor {
 			if (this.getText().length === 0) {
 				const handler = this.onCtrlD ?? this.actionHandlers.get("exit");
 				if (handler) handler();
+				return;
 			}
-			return; // Always consume
+			// Fall through to editor handling for delete-char-forward when not empty
 		}
 
 		// Check all other app actions