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.
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