feat(coding-agent): add ctx.ui.setEditorComponent() extension API

- Add setEditorComponent() to ctx.ui for custom editor components
- Add CustomEditor base class for extensions (handles app keybindings)
- Add keybindings parameter to ctx.ui.custom() factory (breaking change)
- Add modal-editor.ts example (vim-like modes)
- Add rainbow-editor.ts example (animated text highlighting)
- Update docs: extensions.md, tui.md Pattern 7
- Clean up terminal on TUI render errors

packages/coding-agent/examples/extensions/snake.ts

@@ -327,7 +327,7 @@ export default function (pi: ExtensionAPI) {
 				}
 			}
 
-			await ctx.ui.custom((tui, _theme, done) => {
+			await ctx.ui.custom((tui, _theme, _kb, done) => {
 				return new SnakeComponent(
 					tui,
 					() => done(undefined),