feat(cron): support Discord thread_id in deliver targets

Add Discord thread support to cron delivery and send_message_tool.

- _parse_target_ref: handle discord platform with chat_id:thread_id format
- _send_discord: add thread_id param, route to /channels/{thread_id}/messages
- _send_to_platform: pass thread_id through for Discord
- Discord adapter send(): read thread_id from metadata for gateway path
- Update tool schema description to document Discord thread targets

Cherry-picked from PR #7046 by pandacooming (maxyangcn).

Follow-up fixes:
- Restore proxy support (resolve_proxy_url/proxy_kwargs_for_aiohttp) that was
  accidentally deleted — would have caused NameError at runtime
- Remove duplicate _DISCORD_TARGET_RE regex; reuse existing _TELEGRAM_TOPIC_TARGET_RE
  via _NUMERIC_TOPIC_RE alias (identical pattern)
- Fix misleading test comments about Discord negative snowflake IDs
  (Discord uses positive snowflakes; negative IDs are a Telegram convention)
- Rewrite misleading scheduler test that claimed to exercise home channel
  fallback but actually tested the explicit platform:chat_id parsing path

gateway/platforms/discord.py

@@ -770,18 +770,34 @@ class DiscordAdapter(BasePlatformAdapter):
         reply_to: Optional[str] = None,
         metadata: Optional[Dict[str, Any]] = None
     ) -> SendResult:
-        """Send a message to a Discord channel."""
+        """Send a message to a Discord channel or thread.
+
+        When metadata contains a thread_id, the message is sent to that
+        thread instead of the parent channel identified by chat_id.
+        """
         if not self._client:
             return SendResult(success=False, error="Not connected")
 
         try:
-            # Get the channel
-            channel = self._client.get_channel(int(chat_id))
-            if not channel:
-                channel = await self._client.fetch_channel(int(chat_id))
+            # Determine target channel: thread_id in metadata takes precedence.
+            thread_id = None
+            if metadata and metadata.get("thread_id"):
+                thread_id = metadata["thread_id"]
 
-            if not channel:
-                return SendResult(success=False, error=f"Channel {chat_id} not found")
+            if thread_id:
+                # Fetch the thread directly — threads are addressed by their own ID.
+                channel = self._client.get_channel(int(thread_id))
+                if not channel:
+                    channel = await self._client.fetch_channel(int(thread_id))
+                if not channel:
+                    return SendResult(success=False, error=f"Thread {thread_id} not found")
+            else:
+                # Get the parent channel
+                channel = self._client.get_channel(int(chat_id))
+                if not channel:
+                    channel = await self._client.fetch_channel(int(chat_id))
+                if not channel:
+                    return SendResult(success=False, error=f"Channel {chat_id} not found")
 
             # Format and split message if needed
             formatted = self.format_message(content)