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.go

@@ -3,8 +3,13 @@
 // new comment from someone other than the agent, its CI fails, or it loses
 // mergeability after the baseline. Benign transitions (CI pending→success, the
 // agent's own pushes and comments) are ignored, and so is any condition that was
-// already true at the baseline -- which watchpr prints, so a run started against
-// a conflicted or CI-red PR says so.
+// already true at the baseline -- which watchpr prints on stderr (as a JSON
+// record under --json), so a run started against a conflicted or CI-red PR says
+// so. One case is never reported at all: a conflict introduced by the push
+// immediately before the watch started, on a PR whose head and base then never
+// move, stays silent for the whole run, because nothing in Gitea's payload
+// separates it from a merge check still in flight. The baseline line is the
+// only notice of it.
 //
 //	watchpr owner/repo#12 owner/repo:15
 //	watchpr --once --json owner/repo#12
@@ -15,6 +20,7 @@ package main
 import (
 	"encoding/json"
 	"fmt"
+	"io"
 	"os"
 	"strings"
 	"time"
@@ -131,13 +137,7 @@ func runWatch(c *agent.GiteaClient, refs []agent.PRRef, interval time.Duration,
 	defer ticker.Stop()
 
 	onBaseline := func(states []agent.PRState) {
-		if jsonMode {
-			return
-		}
-		fmt.Fprintf(os.Stderr, "watching %d PR(s) every %s; baseline established\n", len(refs), interval)
-		for _, st := range states {
-			fmt.Fprintln(os.Stderr, baselineLine(st))
-		}
+		emitBaselines(os.Stderr, states, interval, jsonMode)
 	}
 	onError := func(ref agent.PRRef, err error) {
 		fmt.Fprintf(os.Stderr, "warning: polling %s: %v\n", ref.String(), err)
@@ -151,6 +151,32 @@ func runWatch(c *agent.GiteaClient, refs []agent.PRRef, interval time.Duration,
 	return nil
 }
 
+// baselineRecord is the --json form of baselineLine. Both go to stderr, leaving
+// the stdout contract a single result record: a caller automating watchpr is
+// precisely the one who needs to be told the watch started against a PR that is
+// already conflicted, since no alert will ever follow for it.
+type baselineRecord struct {
+	Baseline   bool          `json:"baseline"`
+	Suppressed string        `json:"suppressed,omitempty"`
+	State      agent.PRState `json:"state"`
+}
+
+// emitBaselines writes the state each watch started from, in whichever form the
+// caller asked for.
+func emitBaselines(w io.Writer, states []agent.PRState, interval time.Duration, jsonMode bool) {
+	if jsonMode {
+		enc := json.NewEncoder(w)
+		for _, st := range states {
+			_ = enc.Encode(baselineRecord{Baseline: true, Suppressed: suppressedAtBaseline(st), State: st})
+		}
+		return
+	}
+	_, _ = fmt.Fprintf(w, "watching %d PR(s) every %s; baseline established\n", len(states), interval)
+	for _, st := range states {
+		_, _ = fmt.Fprintln(w, baselineLine(st))
+	}
+}
+
 // describeFailure names the cause of a terminal failure so a watcher that stops
 // says why. An anonymous rejection, a permission boundary and a token that
 // outlived its Vault lease are three different problems and only the last is