Fix loading bar persisting on screen after task completion

[nickwesselman]

Summary

• Ink 6's unmount flow renders a final frame before the React tree clears, leaving stale loading bar output (from renderTasks and renderSingleTask) visible after task completion
• Adds an eraseOnExit option to the render() wrapper that intercepts stdout writes to track the maximum output height, then erases that many lines after the Ink instance exits
• Applies eraseOnExit: true to renderTasks and renderSingleTask

Test plan

• All 40 existing tests pass (ui.test.ts, SingleTask.test.tsx, Tasks.test.tsx)
• Run pnpm shopify kitchen-sink async and verify loading bars disappear after each task completes
• Verify renderConcurrent output (which should NOT erase) still works correctly

🤖 Generated with Claude Code

[github-actions]

Coverage report

St.❔	Category	Percentage	Covered / Total
🟢	Statements	82.23%	14945/18174
🟡	Branches	74.67%	7408/9921
🟢	Functions	81.36%	3768/4631
🟢	Lines	82.63%	14132/17103

Test suite run success

3910 tests passing in 1502 suites.

Report generated by 🧪jest coverage report action from ce16c61

[github-actions]

Differences in type declarations

We detected differences in the type declarations generated by Typescript for this branch compared to the baseline ('main' branch). Please, review them to ensure they are backward-compatible. Here are some important things to keep in mind:

• Some seemingly private modules might be re-exported through public modules.
• If the branch is behind main you might see odd diffs, rebase main into this branch.

New type declarations

We found no new type declarations in this PR

Existing type declarations

packages/cli-kit/dist/private/node/ui.d.ts

@@ -7,7 +7,10 @@ interface RenderOnceOptions {
     renderOptions?: RenderOptions;
 }
 export declare function renderOnce(element: JSX.Element, { logLevel, renderOptions }: RenderOnceOptions): string | undefined;
-export declare function render(element: JSX.Element, options?: RenderOptions): Promise<unknown>;
+interface ExtendedRenderOptions extends RenderOptions {
+    eraseOnExit?: boolean;
+}
+export declare function render(element: JSX.Element, options?: ExtendedRenderOptions): Promise<unknown>;
 export declare class Stdout extends EventEmitter {
     columns: number;
     rows: number;