feat: support query single command description

src/main/kotlin/cn/rtast/fancybot/commands/HelpCommand.kt

@@ -22,6 +22,7 @@ class HelpCommand : BaseCommand() {
     override val commandNames = listOf("/help", "/帮助")
 
     override suspend fun executeGroup(listener: OneBotListener, message: GroupMessage, args: List<String>) {
+        if (args.isEmpty()) {
             val node = NodeMessageChain.Builder()
             val msg = MessageChain.Builder()
             commands.sortedBy { it::class.simpleName }.forEach {
@@ -35,5 +36,24 @@ class HelpCommand : BaseCommand() {
             msg.addText("共计${commands.size}条命令")
             node.addMessageChain(msg.build(), message.sender.userId)
             message.reply(node.build())
+        } else {
+            val commandName = args.first().replace("/", "")
+            val matchedCommand = commands.find {
+                it.commandNames.map { map -> map.replace("/", "") }.contains(commandName)
+            }
+            if (matchedCommand == null) {
+                message.reply("未查询到该命令")
+                return
+            }
+            val description = matchedCommand::class.findAnnotation<CommandDescription>()?.description ?: "暂无描述"
+            val msg = MessageChain.Builder()
+                .addText("命令名称: ${matchedCommand::class.simpleName}")
+                .addNewLine()
+                .addText("指令别名: ${matchedCommand.commandNames.joinToString(",")}")
+                .addNewLine()
+                .addText("指令描述: $description")
+                .build()
+            message.reply(msg)
+        }
     }
 }