Add Decision, decide and the runner stub to brokerd

Implemented-By: Laguna S 2.1 (OpenCode)

crates/brokerd/src/runner.rs

@@ -0,0 +1,12 @@
+//! Tool runner stub. Real tool execution arrives in M3.
+
+use crate::policy::Decision;
+use proto::ToolResponse;
+
+/// Takes the Decision by value, so one decision cannot run a tool twice.
+pub fn run(decision: Decision) -> ToolResponse {
+    let _ = decision;
+    ToolResponse::Failed {
+        message: "no tool runner until M3".to_string(),
+    }
+}