Documentation MCP server — JSON-RPC 2.0 endpoint (no auth)

The read-only documentation server: search_docs, get_doc and list_docs over the same /docs corpus llms-full.txt inlines, served as JSON-RPC 2.0 with the same envelope conventions as /api/mcp. Unauthenticated by design — it can only return text that is already published at /docs, and it holds no tenant data and accepts no writes. Retrieval is keyword overlap over a corpus frozen at build time; no model runs in this path.