Remove duplicate truncate functions, reuse utils.Truncate

Multiple packages had their own private truncate implementations:
- channels/telegram.go: truncateString (byte-based, no "...")
- channels/dingtalk.go: truncateStringDingTalk (byte-based, no "...")
- voice/transcriber.go: truncateText (byte-based, with "...")

All three are functionally equivalent to the existing utils.Truncate,
which already handles rune-safe truncation and appends "..." correctly.

Replace all private copies with utils.Truncate and delete the dead code.

pkg/channels/feishu.go

@@ -15,6 +15,7 @@ import (
 	"github.com/sipeed/picoclaw/pkg/bus"
 	"github.com/sipeed/picoclaw/pkg/config"
 	"github.com/sipeed/picoclaw/pkg/logger"
+	"github.com/sipeed/picoclaw/pkg/utils"
 )
 
 type FeishuChannel struct {
@@ -165,7 +166,7 @@ func (c *FeishuChannel) handleMessageReceive(_ context.Context, event *larkim.P2
 	logger.InfoCF("feishu", "Feishu message received", map[string]interface{}{
 		"sender_id": senderID,
 		"chat_id":   chatID,
-		"preview":   truncateString(content, 80),
+		"preview":   utils.Truncate(content, 80),
 	})
 
 	c.HandleMessage(senderID, chatID, content, nil, metadata)