watchpr: emit the baseline under --json

The baseline is the only notice that a watch started against an already
conflicted PR, and it was missing from the mode automation uses. It goes to
stderr as a JSON record, leaving stdout a single result record.

cmd/watchpr/main_test.go

@@ -1,6 +1,8 @@
 package main
 
 import (
+	"bytes"
+	"encoding/json"
 	"errors"
 	"fmt"
 	"io"
@@ -8,6 +10,7 @@ import (
 	"net/http/httptest"
 	"strings"
 	"testing"
+	"time"
 
 	"git.unkin.net/unkin/agent-tools/internal/agent"
 )
@@ -212,3 +215,44 @@ func TestBaselineLineNamesSuppressedConditions(t *testing.T) {
 		t.Errorf("baselineLine = %q, want no suppression note for a clean baseline", got)
 	}
 }
+
+// --json is the mode automation uses, and automation is exactly who needs to be
+// told the watch started against an already-conflicted PR -- the one case that
+// will never produce an alert. It must therefore be emitted in JSON mode too,
+// on stderr, where it cannot corrupt the result record on stdout.
+func TestBaselineIsEmittedInJSONMode(t *testing.T) {
+	st := agent.PRState{
+		Ref:       agent.PRRef{Owner: "unkin", Repo: "repo", Number: 7},
+		State:     "open",
+		Mergeable: agent.MergeNo,
+		CIStatus:  "failure",
+		HeadSHA:   "cafebabecafebabe",
+	}
+	var buf bytes.Buffer
+	emitBaselines(&buf, []agent.PRState{st}, 30*time.Second, true)
+
+	var got baselineRecord
+	if err := json.Unmarshal(buf.Bytes(), &got); err != nil {
+		t.Fatalf("baseline is not a JSON record (%v); got %q", err, buf.String())
+	}
+	if !got.Baseline {
+		t.Error("record does not mark itself as the baseline")
+	}
+	if got.State.Ref != st.Ref || got.State.Mergeable != agent.MergeNo {
+		t.Errorf("record state = %+v, want the conflicted snapshot the watch started from", got.State)
+	}
+	for _, want := range []string{"already non-mergeable", "CI already failure"} {
+		if !strings.Contains(got.Suppressed, want) {
+			t.Errorf("suppressed = %q, want it to mention %q", got.Suppressed, want)
+		}
+	}
+
+	buf.Reset()
+	clean := st
+	clean.Mergeable = agent.MergeYes
+	clean.CIStatus = "success"
+	emitBaselines(&buf, []agent.PRState{clean}, 30*time.Second, true)
+	if strings.Contains(buf.String(), "suppressed") {
+		t.Errorf("clean baseline = %q, want no suppression field", buf.String())
+	}
+}