The session store was copying the ENTIRE parent DM transcript into new
thread sessions. This caused unrelated conversations to bleed across
threads in Slack DMs.
The Slack adapter already handles thread context correctly via
_fetch_thread_context() (conversations.replies API), which fetches
only the actual thread messages. The session-level seeding was both
redundant and harmful.
No other platform (Telegram, Discord) uses DM threads, so the seeding
code path was only triggered by Slack — where it conflicted with the
adapter-level context.
Tests updated to assert thread isolation: all thread sessions start
empty, platform adapters are responsible for injecting thread context.
Salvage of PR #5868 (jarvisxyz). Reported by norbert on Discord.
