feat(progress): Annotate -- a note on a step, saved at once (second review finding 1)

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

internal/progress/tracker.go

@@ -186,6 +186,20 @@ func (t *Tracker) Finish() {
 	t.save()
 }
 
+// Annotate attaches a short plain note to a phase and saves it at once. It is for a
+// remark the reader should see although nothing failed. Durations and counts only: a note
+// is shown on a page about a private recording.
+func (t *Tracker) Annotate(key, note string) {
+	t.mu.Lock()
+	defer t.mu.Unlock()
+	s := t.find(key)
+	if s == nil {
+		return
+	}
+	s.Note = note
+	t.save()
+}
+
 // Heartbeat stamps the job's row every beatEvery and logs which phase is still running
 // every stillEvery, until ctx ends. It runs on its own, independent of any engine call:
 // a heartbeat that stopped means the process is gone, not that a request is slow.