feat(hooks): add setWidget API for multi-line status displays

- ctx.ui.setWidget(key, lines) for multi-line displays above editor
- Widgets appear below 'Working...' indicator, above editor
- Supports ANSI styling including strikethrough
- Added theme.strikethrough() method
- Plan-mode hook now shows todo list with checkboxes
- Completed items show checked box and strikethrough text

packages/coding-agent/src/modes/interactive/theme/theme.ts

@@ -376,6 +376,10 @@ export class Theme {
 		return chalk.inverse(text);
 	}
 
+	strikethrough(text: string): string {
+		return chalk.strikethrough(text);
+	}
+
 	getFgAnsi(color: ThemeColor): string {
 		const ansi = this.fgColors.get(color);
 		if (!ansi) throw new Error(`Unknown theme color: ${color}`);