mesh-sdk for Rust

The skeleton (novox/hq ADR 0188): mesh_sdk::serve(name, &tools) speaks MCP over stdio to the node's tool runtime. A Tool is a name, a description, an argument schema and a function. A tool named <seat>.<verb> is the seat's implementation. One dependency, serde_json.

What it holds, under ADR 0039's test: the stdio loop and the tool type. Nothing else — no bus, no client, nothing of any module's. cargo build --example greet and pipe a tools/list request in.