Add MCP server to the worker: integrations.sh/mcp (detect tool)

Write-once tool registry (Effect v4) projected to MCP, served from the
existing Cloudflare Worker at /mcp. First tool: detect — backed by the same
detect() engine the REST /api/<domain>/detect route uses. PostHog tracking,
the static site, and the detect route are untouched. nodejs_compat enabled.

Author: RhysSullivan