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_test.go

@@ -0,0 +1,20 @@
+package router
+
+import (
+	"strings"
+	"testing"
+)
+
+func TestKnowledgePrompt(t *testing.T) {
+	prompt := KnowledgePrompt()
+	if prompt == "" {
+		t.Fatal("KnowledgePrompt returned empty string")
+	}
+	// Must contain key instructions
+	checks := []string{"Мавена", "не знаю", "не выдумывай"}
+	for _, c := range checks {
+		if !strings.Contains(strings.ToLower(prompt), strings.ToLower(c)) {
+			t.Errorf("KnowledgePrompt should mention %q", c)
+		}
+	}
+}