Add Cargo workspace, crate skeletons and the gate

Implemented-By: Laguna S 2.1 (OpenCode)

crates/brokerd/Cargo.toml

@@ -0,0 +1,12 @@
+[package]
+name = "brokerd"
+version = "0.1.0"
+edition.workspace = true
+rust-version.workspace = true
+publish.workspace = true
+
+[lints]
+workspace = true
+
+[dependencies]
+proto.workspace = true

crates/brokerd/src/lib.rs

@@ -0,0 +1 @@
+//! The broker: the only role that holds authority.

crates/brokerd/src/main.rs

@@ -0,0 +1,4 @@
+fn main() {
+    eprintln!("brokerd: not implemented until M3");
+    std::process::exit(2);
+}