feat(cli): JSON output envelope with stable error codes

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

internal/cli/envelope_test.go

@@ -0,0 +1,43 @@
+package cli
+
+import (
+	"bytes"
+	"encoding/json"
+	"testing"
+)
+
+func TestSuccessEnvelope(t *testing.T) {
+	var buf bytes.Buffer
+	if err := Success(map[string]any{"count": 2}).Write(&buf); err != nil {
+		t.Fatalf("write: %v", err)
+	}
+	var got map[string]any
+	if err := json.Unmarshal(buf.Bytes(), &got); err != nil {
+		t.Fatalf("unmarshal: %v", err)
+	}
+	if got["error"] != false {
+		t.Fatalf("error should be false: %v", got["error"])
+	}
+	if _, ok := got["data"]; !ok {
+		t.Fatal("data key missing")
+	}
+	if ed, ok := got["error_detail"].(map[string]any); !ok || len(ed) != 0 {
+		t.Fatalf("error_detail should be empty object: %v", got["error_detail"])
+	}
+}
+
+func TestFailureEnvelope(t *testing.T) {
+	var buf bytes.Buffer
+	_ = Failure(CodeNotFound, "uid 7 not found").Write(&buf)
+	var got struct {
+		Error       bool `json:"error"`
+		ErrorDetail struct {
+			Code    string `json:"code"`
+			Message string `json:"message"`
+		} `json:"error_detail"`
+	}
+	_ = json.Unmarshal(buf.Bytes(), &got)
+	if !got.Error || got.ErrorDetail.Code != "not_found" {
+		t.Fatalf("bad failure envelope: %+v", got)
+	}
+}