Let the LLM router answer "unknown" when it cannot route

Chose an 8th enum value over a confidence number: the model already picks
one enum token, so it costs nothing in the grammar, while a score from a
0.8B model would be uncalibrated noise. A refusal returns "no decision"
with no error, which is the fall-through the caller already uses for a
bad parse, so the classifier and its clarify gate take the turn.

Reviewers: the prompt's counter-examples matter most — a small model will
over-use any easy escape hatch. The training workspace copy of the prompt
still needs the same edit (Vikunja #362).

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CGeSZxh1DCtRxmFVSYVGvJ

internal/router/llmrouter_test.go

@@ -100,8 +100,10 @@ func TestLLMRouterReminderMapping(t *testing.T) {
 	}
 }
 
+// An intent name that is not in the contract at all (as opposed to "unknown",
+// which is a real refusal) still defaults to chat.
 func TestLLMRouterChatFallback(t *testing.T) {
-	lr := NewLLMRouter(mockLLM{out: `{"intent":"unknown"}`})
+	lr := NewLLMRouter(mockLLM{out: `{"intent":"banana"}`})
 	d, ok, err := lr.Route(context.Background(), "как дела?", time.Now())
 	if err != nil || !ok {
 		t.Fatalf("ok=%v err=%v", ok, err)
@@ -111,6 +113,61 @@ func TestLLMRouterChatFallback(t *testing.T) {
 	}
 }
 
+// The model must be able to say "I could not route this".
+func TestRouteGrammarAllowsUnknown(t *testing.T) {
+	if !strings.Contains(routeGrammar, `"\"unknown\""`) {
+		t.Fatal("grammar cannot express a refusal")
+	}
+}
+
+// If the prompt does not tell the model when to refuse, it never will.
+func TestRoutePromptExplainsUnknown(t *testing.T) {
+	if !strings.Contains(routeSystem, "unknown") {
+		t.Fatal("prompt never mentions the unknown intent")
+	}
+	if !strings.Contains(routeSystem, `"сделай это" → {"intent":"unknown"}`) {
+		t.Fatal("prompt lost its worked refusal example")
+	}
+	// A refusal-only router is useless, so the prompt must also show cases that
+	// look ambiguous but are not.
+	if !strings.Contains(routeSystem, "здесь unknown не нужен") {
+		t.Fatal("prompt lost its counter-examples")
+	}
+}
+
+// A refusal is not an error. It reports "no decision" so the cascade moves on.
+func TestLLMRouterUnknownRefuses(t *testing.T) {
+	lr := NewLLMRouter(mockLLM{out: `{"intent":"unknown"}`})
+	_, ok, err := lr.Route(context.Background(), "сделай это", time.Now())
+	if ok {
+		t.Fatal("a refusal must not produce a usable decision")
+	}
+	if err != nil {
+		t.Fatalf("a refusal is not an error, got %v", err)
+	}
+}
+
+// The whole point of the refusal: the turn keeps going on the classifier, the
+// same way it does when the model returns garbage.
+func TestRouterFallsBackWhenLLMRefuses(t *testing.T) {
+	c := NewClassifier(NewHashEmbedder(1024))
+	seedClassifier(t, c)
+	r := New(Config{
+		Classifier: c,
+		Extractor:  Extractor{Time: StubDateTimeParser{}, Facts: DefaultFactParser{}},
+		Threshold:  0.4,
+		LLM:        NewLLMRouter(mockLLM{out: `{"intent":"unknown"}`}),
+	})
+	d, err := r.Route(context.Background(), "напомни позвонить маме", refNow())
+	if err != nil {
+		t.Fatalf("route: %v", err)
+	}
+	// Stage 1 is the LLM's own answer; the classifier lands on stage 2 or 3.
+	if d.Stage < 2 {
+		t.Fatalf("want the classifier to decide, got stage %d (%+v)", d.Stage, d)
+	}
+}
+
 func TestLLMRouterLLMError(t *testing.T) {
 	lr := NewLLMRouter(mockLLM{out: "", err: fmt.Errorf("llm down")})
 	_, ok, err := lr.Route(context.Background(), "x", time.Now())