Detailed MCP-transport design for the stored-query/MCP work, building on the
shipped #128 registry. Corrects the draft against the branch head: the coarse
invoke_query gate + 404 denial-masking are already wired (server_invoke_query),
so per-query invoke_query scope (PolicyRequest has no query-name dimension yet)
is the real prerequisite; positions the doc as superseding rfc-001's MCP
transport (/mcp/tools+/mcp/invoke) and reconciles the shipped mcp.expose YAML
form and the schema-introspection non-goal; grounds the parity surface in the
actual omnigraph-ts package (13 tools with read/change ids, 2 resources).
