feat(config): allow placeholder text to be string or list

Allow PlaceholderConfig.Text to accept either a single string or an
array of strings, from which one is randomly selected at runtime.
This maintains backward compatibility with existing single-string configs
while enabling random placeholder selection.

Changes:
- Modify PlaceholderConfig.Text type from string to FlexibleStringSlice
- Add GetRandomText() helper method for random selection
- Update SendPlaceholder in all channels to use GetRandomText()
- Update config.example.json with array placeholder examples
- Update Matrix channel documentation

pkg/channels/feishu/feishu_64.go

@@ -211,10 +211,7 @@ func (c *FeishuChannel) SendPlaceholder(ctx context.Context, chatID string) (str
 		return "", nil
 	}
 
-	text := c.config.Placeholder.Text
-	if text == "" {
-		text = "Thinking..."
-	}
+	text := c.config.Placeholder.GetRandomText()
 
 	cardContent, err := buildMarkdownCard(text)
 	if err != nil {