router: a reminder with no subject asks instead of guessing (V-383)

Slots.Text was the raw utterance for every intent, so a reminder could not
have an empty subject. StillMissing never reported SlotText, the question
"О чём напомнить?" was unaskable, and the branch in PendingQuestion.Answer
that fills a text slot could only overwrite the whole request.

The LLM path now keeps the model's own text, empty included, and the gate
turns a subjectless reminder into a question. The classifier path is
unchanged: it has no subject parser, so the utterance is the only signal it
has.

cmd/mavend/clarify_test.go

@@ -465,3 +465,39 @@ func TestExpiryNoticeSurvivesAConfirmTurn(t *testing.T) {
 		t.Fatal("the expired question must be gone")
 	}
 }
+
+// The other half of the subject question: his answer must fill the empty slot,
+// not replace the request. Slots.Text used to be the whole raw utterance for
+// every intent, so the branch that fills a text slot could only ever overwrite
+// (Vikunja #383). Here the parked request holds the hour and the answer holds
+// what to say at it, and the reminder that lands has both.
+func TestClarifySubjectAnswerFillsRatherThanClobbers(t *testing.T) {
+	ctx := context.Background()
+	h, st, _ := newClarifyHandler(t)
+	at := h.now().Add(2 * time.Hour)
+
+	question, asked := h.askClarify(clarifyDec(router.IntentReminder,
+		router.Slots{Time: at, HasTime: true}, "напомни в 11"))
+	if !asked || question != "О чём напомнить?" {
+		t.Fatalf("expected the subject question, got %q asked=%v", question, asked)
+	}
+
+	reply, handled := h.resolveClarifyAnswer(ctx, "позвонить маме")
+	if !handled {
+		t.Fatal("the answer to an open question must be consumed as an answer")
+	}
+	if reply == clarifyGaveUp {
+		t.Fatalf("a good answer must not drop the request: %q", reply)
+	}
+
+	reminders, err := st.DueReminders(ctx, h.now().Add(48*time.Hour))
+	if err != nil || len(reminders) != 1 {
+		t.Fatalf("clarified reminder was not created: reminders=%v err=%v", reminders, err)
+	}
+	if !strings.Contains(reminders[0].Payload, "маме") {
+		t.Fatalf("the answer never reached the reminder: %q", reminders[0].Payload)
+	}
+	if !strings.Contains(reminders[0].Payload, "11") {
+		t.Fatalf("the answer clobbered the original request: %q", reminders[0].Payload)
+	}
+}