brokerd: the refusal without a runner names the missing [runner] section

Implemented-By: OpenCode session (model recorded in docs/implementer-log.md)

crates/brokerd/src/runner.rs

@@ -95,7 +95,7 @@ impl Runtime for Refusing {
 }
 
 /// The sentence `Refusing` puts in every `Unavailable`: fixed, because it reaches the model.
-pub const REFUSING: &str = "the runner arrives in M3b";
+pub const REFUSING: &str = "no tool runner is configured: brokerd.toml has no [runner] section";
 
 /// Turn an allowed call into a `RunSpec`, run it, and answer. A `RunError`'s text reaches the
 /// model unchanged, so a runtime may only put a fixed sentence in it.

crates/brokerd/tests/runner.rs

@@ -161,7 +161,10 @@ fn a_run_error_is_a_failure_with_the_runtimes_sentence() {
 
 #[test]
 fn the_production_runtime_refuses_every_call() {
-    assert_eq!(REFUSING, "the runner arrives in M3b");
+    assert_eq!(
+        REFUSING,
+        "no tool runner is configured: brokerd.toml has no [runner] section"
+    );
     let d = allowed(
         vec![grant("n", "read_file", Mode::Auto).paths(&["/n"])],
         read("/n/a"),