Add Bilig WorkPaper formula readback tool

[gregkonush]

Summary

• Add an Open WebUI Tools implementation for Bilig WorkPaper formula readback.
• Include a short README with install steps, default endpoint, and a sample prompt.
• Keep the tool dependency-light: Python stdlib HTTP plus Pydantic valves.

Why

This gives Open WebUI users a small spreadsheet-backed calculation tool: write one input cell, recalculate formulas through a Bilig WorkPaper endpoint, and return verified JSON readback. The default endpoint uses the public Bilig forecast demo, and valves let admins point it at their own Bilig service.

Bilig also has a first-party Open WebUI setup page for the broader agent path, including native Streamable HTTP MCP and mcpo bridge options:
https://proompteng.github.io/bilig/open-webui-workpaper-mcp.html

Validation

python3 -m py_compile functions/tools/bilig_workpaper_formula_readback/main.py
git diff --check -- functions/tools/bilig_workpaper_formula_readback
rg -n "bilig|Bilig|WorkPaper" functions/tools/bilig_workpaper_formula_readback -S
npm view @bilig/workpaper version
npm exec --yes --package @bilig/workpaper@latest -- bilig-evaluate --door agent-mcp --json

npm view @bilig/workpaper version returned 0.154.0.

The current public Bilig evaluator returned verified: true with package versions @bilig/workpaper@0.154.0 and xlsx-formula-recalc@0.154.0. It verifies tool discovery, Inputs!B3 mutation, formula readback 60000 -> 96000, JSON persistence, restored readback, and restart readback.

Original local smoke against the Open WebUI tool endpoint path also returned verified=true, editedCell=Inputs!B3, and after.expectedArr=120000 after setting Inputs!B3 to 0.5.