codeSplitters: an MCP server on the Worker, built from the API reference #384

Follow-up to #379, which shipped the API reference (GET /api, src/api.ts). The other option from that issue: a remote MCP server on the Worker (say /mcp) whose tools come from the same reference, so agents outside the browser can use codeSplitters without hand-writing HTTP calls. Needs a way to sign in an MCP client (an API token, or OAuth through Better Auth).