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.

internal/router/llmrouter.go

@@ -210,7 +210,11 @@ func (lr *LLMRouter) Route(ctx context.Context, utterance string, now time.Time)
 		d.Slots.HasKey = a.Key != ""
 	case IntentReminder:
 		d.Intent = IntentReminder
-		d.Slots.Text = firstNonEmpty(a.Text, utterance)
+		// No utterance fallback here, unlike every other intent below. The
+		// model returning no text for a reminder means it found no subject,
+		// and "напомни в 11" is not a subject. Leaving Text empty is what
+		// lets the gate turn that into a question (Vikunja #383).
+		d.Slots.Text = a.Text
 	case IntentNote:
 		d.Intent = IntentNote
 		d.Slots.Text = firstNonEmpty(a.Text, utterance)