Top-level channel messages were creating isolated per-message sessions because roomThreadId fell through to threadContext.messageTs whenever replyToMode was not off.
Introduced in #10686, every new channel message got its own session key (agent:...🧵<messageTs>), breaking conversation continuity.
Fix: only derive thread-specific session keys for actual thread replies. Top-level channel messages stay on the per-channel session key regardless of replyToMode.
Fixes#32285