phraser: say "даже не знаю, что сказать" when there is nothing to say (V-397)

Review of #108: "поговорили." reads as a summary of a conversation that did
not happen. One exported constant now, so the Stub, the LLMPhraser fallback
and the daemon all say the same thing.

internal/voice/replier.go keeps its own copy — that is the separate replier
seam, not this one.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

cmd/mavend/actions.go

@@ -40,6 +40,7 @@ import (
 	"context"
 	"log"
 
+	"github.com/kami/maven/internal/phraser"
 	"github.com/kami/maven/internal/router"
 )
 
@@ -65,7 +66,7 @@ func (h *reactiveHandler) actionChat(ctx context.Context, dec router.Decision) s
 		log.Printf("voice: chat: %v", err)
 	}
 	if reply == "" {
-		return "поговорили."
+		return phraser.ChatFallback
 	}
 	return reply
 }