…claw#26849)

* feat(slack): create thread sessions for auto-threaded DM messages

When replyToMode="all", every top-level message starts a new Slack thread.
Previously, only subsequent replies in that thread got an isolated session
(via 🧵<threadTs> suffix). The initial message fell back to the base
DM session, mixing context across unrelated conversations.

Now, when replyToMode="all" and a message is not already a thread reply,
the message's own ts is used as the threadId for session key resolution.
This gives the initial message AND all subsequent thread replies the same
isolated session.

This enables per-thread session isolation for Slack DMs — each new message
starts its own thread and session, keeping conversations separate.

* Slack: fix auto-thread session key mode check and add changelog

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Co-authored-by: Tak Hoffman <[email protected]>
(cherry picked from commit 93ac2b4)