docs(agent): add AI SDK onStepFinish WorkPaper transcript

[gregkonush]

Goal

Add a small, copy-pasteable transcript that shows how an app can use the Vercel AI SDK onStepFinish callback to record WorkPaper tool calls, tool results, and the final formula readback.

Why this helps

The repository now has runnable AI SDK generateText() and streamText() smokes. A short onStepFinish transcript would help agent builders see where to log or persist each verified WorkPaper step without changing the core tool implementation.

Suggested files

• docs/vercel-ai-sdk-langchain-spreadsheet-tool.md
• docs/agent-workpaper-tool-calling-recipe.md
• examples/headless-workpaper/README.md

Done criteria

• Include a TypeScript snippet using onStepFinish with step.toolCalls and step.toolResults.
• Keep the WorkPaper proof specific: Inputs!B3, before ARR 60000, after ARR 96000, and restore checks.
• Link to the existing runnable smokes:
  • examples/headless-workpaper/ai-sdk-generate-text-tool-smoke.ts
  • examples/headless-workpaper/ai-sdk-stream-text-tool-smoke.ts
• Run pnpm docs:discovery:check and pnpm format:check.

Scope note

This should be documentation-only unless a tiny helper snippet is clearer than prose. Do not add a provider dependency or API key requirement.