feat(qq): render answers as markdown

Deliver final Harness answers to QQ as msg_type=2 markdown so bold,
headings, code blocks, lists, and tables render natively in QQ.

- add markdown-reply.mjs: chunk long answers at code-fence/table
  boundaries (4500-char limit) and send each chunk with a unique
  msg_seq to avoid passive-reply dedup (40054005)
- fall back to plain text per chunk when the platform rejects
  markdown, so bots without markdown capability keep working
- route the non-stream final answer path in qq-bridge through the
  markdown sender; streaming replies already use markdown content
  type, and command/notice texts stay plain

src/channels/qq/markdown-reply.mjs

@@ -0,0 +1,135 @@
+// QQ markdown 回复投递：长文按代码块/表格边界切分，以 msg_type=2 发送，
+// 平台拒绝 markdown 时逐条回退纯文本。
+
+const DEFAULT_CHUNK_LIMIT = 4_500;
+const CODE_FENCE_OPEN = /^```/;
+const GFM_TABLE_LINE = /^\|.+\|$/;
+
+/**
+ * 按换行边界切分 Markdown 文本：
+ * - 不在代码块中间断开；
+ * - 不在 GFM 表格中间断开；
+ * - 超长行在 limit 处硬切，避免单行超限无法投递。
+ */
+export function chunkMarkdownText(text, limit = DEFAULT_CHUNK_LIMIT) {
+  const value = typeof text === 'string' ? text : '';
+  const bound = Number.isInteger(limit) && limit > 0 ? limit : DEFAULT_CHUNK_LIMIT;
+  if (value.length <= bound) return value ? [value] : [];
+
+  const lines = value.split('\n');
+  const chunks = [];
+  let current = '';
+  let inCodeBlock = false;
+  let tableBuffer = [];
+
+  const appendBlock = (block) => {
+    if (block.length <= bound) {
+      if (!current) {
+        current = block;
+        return;
+      }
+      const candidate = `${current}\n${block}`;
+      if (candidate.length > bound) {
+        chunks.push(current);
+        current = block;
+      } else {
+        current = candidate;
+      }
+      return;
+    }
+    // 超大块：收束当前块后按 bound 硬切，保证每块可投递。
+    if (current) {
+      chunks.push(current);
+      current = '';
+    }
+    let remaining = block;
+    while (remaining.length > bound) {
+      chunks.push(remaining.slice(0, bound));
+      remaining = remaining.slice(bound);
+    }
+    current = remaining;
+  };
+
+  const flushTable = () => {
+    if (tableBuffer.length === 0) return;
+    const block = tableBuffer.join('\n');
+    tableBuffer = [];
+    appendBlock(block);
+  };
+
+  const appendLine = (line) => {
+    let remaining = line;
+    // 超长行先硬切，保证每块不超过 bound。
+    while (remaining.length > bound) {
+      if (current) {
+        chunks.push(current);
+        current = '';
+      }
+      chunks.push(remaining.slice(0, bound));
+      remaining = remaining.slice(bound);
+    }
+    appendBlock(remaining);
+  };
+
+  for (const line of lines) {
+    if (CODE_FENCE_OPEN.test(line)) {
+      flushTable();
+      if (!inCodeBlock && current) {
+        // 代码块开启：先收束当前块，让整个代码块从新块开始。
+        chunks.push(current);
+        current = '';
+      }
+      inCodeBlock = !inCodeBlock;
+      appendLine(line);
+      continue;
+    }
+    if (inCodeBlock) {
+      appendLine(line);
+      continue;
+    }
+    if (GFM_TABLE_LINE.test(line)) {
+      tableBuffer.push(line);
+      continue;
+    }
+    flushTable();
+    appendLine(line);
+  }
+
+  flushTable();
+  if (current) chunks.push(current);
+  return chunks;
+}
+
+function nextMsgSeq() {
+  // 与 SDK getNextMsgSeq 相同的随机策略：被动回复同 msg_id 的多条消息
+  // 各自带不同 msg_seq，避免平台去重（错误码 40054005）。
+  const timePart = Date.now() % 100_000_000;
+  const random = Math.floor(Math.random() * 65_536);
+  return (timePart ^ random) % 65_536;
+}
+
+/**
+ * 以 markdown（msg_type=2）发送回复；单条被平台拒绝时回退纯文本（msg_type=0）。
+ * 返回每条消息的平台响应，供调用方提取 provider message ids。
+ */
+export async function sendMarkdownReply(bot, target, text, { logger } = {}) {
+  const chunks = chunkMarkdownText(text);
+  const results = [];
+  for (const chunk of chunks) {
+    if (typeof bot?.send === 'function') {
+      try {
+        results.push(await bot.send({
+          target,
+          msgType: 2,
+          markdown: { content: chunk },
+          extra: { msg_seq: nextMsgSeq() },
+        }));
+        continue;
+      } catch (error) {
+        logger?.warn?.('[dsh-im:qq] markdown delivery failed; retrying as plain text:', error);
+      }
+    }
+    results.push(await bot.sendText(target, chunk));
+  }
+  return results;
+}

src/channels/qq/qq-bridge.mjs

@@ -37,6 +37,7 @@ import {
   mergeDeliveryReceipts,
   providerMessageIdsFor,
 } from '../shared/semantic/delivery.mjs';
+import { sendMarkdownReply } from './markdown-reply.mjs';
 
 const INTERACTION_RESOLVED_TEXT = '这个问题已在其他客户端处理，无需再次回答。';
 const DEFAULT_FILE_UPLOAD_TIMEOUT_MS = 120_000;
@@ -661,11 +662,13 @@ export class QqHarnessBridge {
           }
         }
         if (!streamFinished) {
-          const sent = await this.#bot.sendText(target, displayAnswer);
+          const deliveries = await sendMarkdownReply(this.#bot, target, displayAnswer, {
+            logger: this.#logger,
+          });
           textReceipt = createDeliveryReceipt({
             deliveryId: messageId,
             presentation: 'qq-text',
-            providerMessageIds: providerMessageIdsFor(sent),
+            providerMessageIds: deliveries.flatMap((delivery) => providerMessageIdsFor(delivery)),
           });
         }
       } catch (error) {