Add docs/CODE-MODE.md recipe

[sajeetharan]

Motivation

Users increasingly ask how to consume MCP servers from code-execution-enabled agent hosts. See Anthropic's Code execution with MCP and Cloudflare Code Mode.

Content outline

• What is Code Mode + when to use it (vs. direct tool calling)
• Cosmos-specific example: "read schema, filter/aggregate in code, upsert selected rows" — parallels the Anthropic GDrive → Salesforce example
• Using the new TypeScript SDK sample (see separate issue)
• How progressive disclosure via search_tools reduces context (see separate issue)
• Non-goals: MCPToolkit does not run agent code — sandboxing / resource limits are the client's responsibility. This is called out explicitly in the Anthropic post.

Acceptance criteria

• New file docs/CODE-MODE.md
• Linked from README.md and docs/USE-CASES.md

Related

Part of the "Code Mode" support umbrella.