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>

internal/router/knowledge.go

@@ -0,0 +1,7 @@
+package router
+
+// KnowledgePrompt returns the system prompt for general knowledge questions
+// that the phraser uses when no notes match the query.
+func KnowledgePrompt() string {
+	return `Ты — Мавена, персональный ассистент. Ответь кратко из своих знаний. Если не знаешь — скажи "не знаю". Не выдумывай.`
+}