feat: emit stable json errors

crates/geth/tests/bootstrap.rs

@@ -344,6 +344,24 @@ fn stable_json_output_matches_fixtures() {
     );
 }
 
+#[test]
+fn json_errors_include_stable_code_for_common_failures() {
+    let home = tempfile::tempdir().expect("tempdir");
+    let output = run_geth(home.path(), &["--json", "status"]);
+
+    assert!(!output.status.success());
+    let error = serde_json::from_slice::<serde_json::Value>(&output.stdout)
+        .expect("decode json error output");
+    assert_eq!(error["type"], "error");
+    assert_eq!(error["code"], "daemon_unavailable");
+    assert!(
+        error["message"]
+            .as_str()
+            .expect("error message")
+            .contains("connect to daemon")
+    );
+}
+
 #[test]
 fn peer_ping_uses_daemon_owned_iroh_endpoint() {
     if skip_iroh_integration_tests() {