docs: add formula workbook custom tool cookbook

[gregkonush]

Summary

Adds a cookbook for exposing spreadsheet-style business logic as a local Composio custom tool using Bilig WorkPaper. The example writes pricing inputs, reads recalculated outputs, and returns JSON persistence proof.

Changes

• Add /cookbooks/formula-workbook-tool
• Add docs/examples/formula-workbook-tool/server.ts
• Link the cookbook from the cookbooks index and metadata

Type of change

• Bug fix
• New feature
• Refactor/Chore
• Documentation
• Breaking change

How Has This Been Tested?

• git diff --check
• bun install --frozen-lockfile
• bun run lint:links
• bun run types:check
• bun run build

Current package sanity check:

npm exec --yes --package @bilig/workpaper@0.107.8 -- bilig-agent-challenge --json

That returned verified: true, changed Inputs!B2, recalculated Summary!B2, exported WorkPaper JSON, and restored matching readback.

Screenshots (if applicable)

N/A

Checklist

• I have read the Code of Conduct and this PR adheres to it
• I ran linters/tests locally and they passed
• I updated documentation as needed
• I added tests or explain why not applicable: docs-only cookbook; docs link/type/build checks passed
• I added a changeset if this change affects published packages: not applicable, docs-only

Additional context

This uses Composio custom tools for local in-process business logic. The workbook package is only installed by the cookbook project; the Composio docs app does not add a new dependency.

The visible Vercel status currently needs Composio team authorization for the preview deployment. The local docs checks above passed before opening the PR.

[vercel]

@gregkonush is attempting to deploy a commit to the Composio Team on Vercel.

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