fix: split Discord messages exceeding 2000-character limit

Discord's API rejects messages longer than 2000 characters with HTTP 400.
Previously, long agent responses were silently lost after retries exhausted.

Adds _split_message() (matching Telegram's approach) to chunk content at
line boundaries before sending. Only the first chunk carries the reply
reference. Retry logic extracted to _send_payload() for reuse across chunks.

Closes #898

nanobot/channels/discord.py

@@ -17,6 +17,27 @@ from nanobot.config.schema import DiscordConfig
 
 DISCORD_API_BASE = "https://discord.com/api/v10"
 MAX_ATTACHMENT_BYTES = 20 * 1024 * 1024  # 20MB
+MAX_MESSAGE_LEN = 2000  # Discord message character limit
+
+
+def _split_message(content: str, max_len: int = MAX_MESSAGE_LEN) -> list[str]:
+    """Split content into chunks within max_len, preferring line breaks."""
+    if len(content) <= max_len:
+        return [content]
+    chunks: list[str] = []
+    while content:
+        if len(content) <= max_len:
+            chunks.append(content)
+            break
+        cut = content[:max_len]
+        pos = cut.rfind('\n')
+        if pos == -1:
+            pos = cut.rfind(' ')
+        if pos == -1:
+            pos = max_len
+        chunks.append(content[:pos])
+        content = content[pos:].lstrip()
+    return chunks
 
 
 class DiscordChannel(BaseChannel):
@@ -79,15 +100,26 @@ class DiscordChannel(BaseChannel):
             return
 
         url = f"{DISCORD_API_BASE}/channels/{msg.chat_id}/messages"
-        payload: dict[str, Any] = {"content": msg.content}
-
-        if msg.reply_to:
-            payload["message_reference"] = {"message_id": msg.reply_to}
-            payload["allowed_mentions"] = {"replied_user": False}
-
         headers = {"Authorization": f"Bot {self.config.token}"}
 
         try:
+            chunks = _split_message(msg.content or "")
+            for i, chunk in enumerate(chunks):
+                payload: dict[str, Any] = {"content": chunk}
+
+                # Only set reply reference on the first chunk
+                if i == 0 and msg.reply_to:
+                    payload["message_reference"] = {"message_id": msg.reply_to}
+                    payload["allowed_mentions"] = {"replied_user": False}
+
+                await self._send_payload(url, headers, payload)
+        finally:
+            await self._stop_typing(msg.chat_id)
+
+    async def _send_payload(
+        self, url: str, headers: dict[str, str], payload: dict[str, Any]
+    ) -> None:
+        """Send a single Discord API payload with retry on rate-limit."""
         for attempt in range(3):
             try:
                 response = await self._http.post(url, headers=headers, json=payload)
@@ -104,8 +136,6 @@ class DiscordChannel(BaseChannel):
                     logger.error("Error sending Discord message: {}", e)
                 else:
                     await asyncio.sleep(1)
-        finally:
-            await self._stop_typing(msg.chat_id)
 
     async def _gateway_loop(self) -> None:
         """Main gateway loop: identify, heartbeat, dispatch events."""