feat(discord): populate guild_id, parent_chat_id, message_id on SessionSource

Discord knows all four identifiers for every inbound message — guild,
channel (or thread), parent channel when in a thread, and the
triggering message.  Pass them into ``SessionSource`` via the new
``build_source()`` kwargs so downstream code (context-prompt builder,
delivery, logging) can use them without re-resolving from discord.py
objects.

For auto-threaded messages, remember the original channel as the
parent before swapping ``chat_id`` to the freshly created thread.

Behavioural: still a no-op — nothing consumes these fields yet.

gateway/platforms/discord.py

@@ -3261,6 +3261,7 @@ class DiscordAdapter(BasePlatformAdapter):
             if auto_thread and not skip_thread and not is_voice_linked_channel and not is_reply_message:
                 thread = await self._auto_create_thread(message)
                 if thread:
+                    parent_channel_id = str(message.channel.id)
                     is_thread = True
                     thread_id = str(thread.id)
                     auto_threaded_channel = thread
@@ -3320,6 +3321,9 @@ class DiscordAdapter(BasePlatformAdapter):
             thread_id=thread_id,
             chat_topic=chat_topic,
             is_bot=getattr(message.author, "bot", False),
+            guild_id=str(message.guild.id) if message.guild else None,
+            parent_chat_id=parent_channel_id,
+            message_id=str(message.id),
         )
 
         # Build media URLs -- download image attachments to local cache so the