[Feature]: WorkPaper formula readback example for spreadsheet agents

### Pre-flight Checklist

- [x] I have searched existing issues and this hasn't been requested yet.

### Problem or Motivation

Spreadsheet agents often update cells through a UI or a file parser, then still have to guess whether dependent formulas actually recalculated. That is risky for pricing, finance checks, import validation, payout logic, and other workflows where the agent needs to show exact readback proof.

AG-UI already has the right primitives for this: a backend agent can run the workbook operation, stream the state/tool-call progress, and let the UI show the verified result instead of treating the spreadsheet grid as the source of truth.

Disclosure: I maintain Bilig WorkPaper. I am opening this first because the contribution guide asks for alignment before significant work.

### Proposed Solution

Would a small community example be a fit for AG-UI?

The example would be a WorkPaper-backed spreadsheet/pricing agent:

1. User asks the agent to change pricing inputs.
2. The backend writes input cells into a Bilig WorkPaper model.
3. Bilig recalculates dependent formulas.
4. The agent returns formula readback, JSON persistence proof, and a `verified` boolean.
5. AG-UI streams the tool run/state update so the frontend can show the proof panel.

I would keep this out of AG-UI core. The useful shape seems like a Dojo/community example or docs recipe that demonstrates AG-UI state/tool events around deterministic workbook calculation.

### Alternatives Considered

- A generic MCP listing: less useful here because it does not show how AG-UI should represent formula readback and state changes.
- A CopilotKit-only recipe: I opened a narrow related PR there for the existing spreadsheet showcase, but AG-UI may deserve a protocol-level example if maintainers think the pattern is useful.
- No upstream example: works for Bilig-owned docs, but it does not help AG-UI users who are trying to model agent-visible spreadsheet state.

### Additional Context

Related prior art:

- CopilotKit spreadsheet recipe PR: https://github.com/CopilotKit/CopilotKit/pull/4999
- Bilig WorkPaper package: https://www.npmjs.com/package/@bilig/workpaper
- Bilig MCP/agent docs: https://proompteng.github.io/bilig/mcp-workpaper-tool-server.html

If this fits, I can send a small PR after maintainer guidance on placement and expected Dojo/test coverage.