maven: general-knowledge phraser routing (task 4)

- Stub.PhraseQuery: empty notes → "не знаю." (was: "no notes")
- LLMPhraser.PhraseQuery: empty notes → general knowledge prompt to LLM
- Voice handler: on notes RAG failure, try phraser before giving up
- New router.KnowledgePrompt() pure function with test

Co-Authored-By: opencode <opencode@anthropic.com>

cmd/mavend/voice.go

@@ -442,7 +442,12 @@ func (h *reactiveHandler) applyAction(ctx context.Context, dec router.Decision)
 		// since(key)==null → don't fire. Tuned for the ONNX embedder; the Hash
 		// floor scores lexically and may rarely clear it.
 		if len(notes) == 0 || notes[0].Score < queryMinScore {
-			return "у меня нет заметок по этому вопросу."
+			// Try general knowledge from the phraser before giving up
+			reply, err := h.phraser.PhraseQuery(ctx, dec.Utterance, nil)
+			if err != nil || reply == "" {
+				return "не знаю."
+			}
+			return reply
 		}
 		texts := make([]string, len(notes))
 		for i, n := range notes {