Add an ADK MCP sample for file-backed formula workbooks

### Problem

ADK has MCP samples, but there is not a focused example showing an agent workflow against a local stdio MCP server that exposes a file-backed, deterministic toolset.

That pattern matters for agent users who want to test tools without requiring a hosted service or an API key, and who need explicit read-after-write verification when the tool can persist local state.

### Proposal

Add a small sample under `contributing/samples/mcp/` that connects ADK to a Bilig WorkPaper MCP server over stdio. The sample should show:

- launching a published stdio MCP server
- reading workbook inputs and formula outputs
- updating an input cell
- verifying recalculated formula readback
- confirming JSON persistence
- running a no-key deterministic smoke path from the CLI

I have a PR open with this docs/sample contribution.