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/test/compaction-hooks.test.ts

@@ -118,6 +118,7 @@ describe.skipIf(!API_KEY)("Compaction hooks", () => {
 				input: async () => undefined,
 				notify: () => {},
 				setStatus: () => {},
+				setWidget: () => {},
 				custom: async () => undefined as never,
 				setEditorText: () => {},
 				getEditorText: () => "",