Define a Golem tool in MoonBit

Annotate an empty struct with #derive.tool and implement commands as public static methods:

/// Echo text.
#derive.tool("echo", version="1.0.0")
struct Echo {}
 
/// Echo one value.
#derive.arg("value", scope="positional")
pub fn Echo::echo(value : String) -> String {
  value
}

Use #derive.command, #derive.arg, #derive.constraint, #derive.result, and #derive.error for explicit command metadata. Custom schema values need #derive.golem_schema. golem build generates registration and typed clients; never edit golem_tool_clients.mbt or other generated files.