[Bug]: Slack thread sessions don't load prior thread messages (including thread parent)

Summary

When a new Slack thread session is created (via replyToMode: "all"), the thread session starts with zero context - it doesn't load any prior messages from the thread, including the thread parent message that started the conversation.

This causes the agent to lose all context mid-conversation when transitioning from channel → thread.

Steps to reproduce

1. Configure Slack with replyToMode: "all" (or similar threading config)
2. In a Slack channel, ask the agent a question (agent responds in channel)
3. Agent's response automatically creates a thread (as expected)
4. Reply to that thread with a follow-up question
5. Observe that the agent has no memory of the conversation - not even its own message that started the thread

Expected behavior

Thread sessions should load all prior thread messages when initialized, including:

• The thread parent message (agent's original response)
• All subsequent messages in the thread

This ensures conversation continuity when OpenClaw creates a new thread session.

Actual behavior

Thread sessions start with empty context. The session key changes from:

• agent:main:slack:channel:<channelId> (main channel)
• → agent:main:slack:channel:<channelId>:thread:<threadTs> (thread session)

But the thread session doesn't load any prior thread messages, even though they exist in Slack.

Example conversation flow:

1. Channel message (me): "Check if this task has been completed: Mobile sidebar"
2. Channel response (agent): "Looking at the Mission Control task list, I can see that task is still in backlog status..." ← thread parent
3. Thread reply (me): "Check the code and see if it was implemented. If not, start on it."
4. Thread response (agent): "I need more context about what you're asking me to check..." ← forgot everything, including its own message

The agent literally forgot its own message (#2) that started the thread.

Environment

• OpenClaw version: 2026.2.3-1
• OS: Windows 11 (10.0.26100)
• Install method: npm global
• Slack config:

  {
    channels: {
      slack: {
        mode: "socket",
        groupPolicy: "open",
        replyToMode: "all"
      }
    }
  }

Additional context

This might be related to issue #758 (thread context inheritance issues), but that issue focuses on threads inheriting context from OTHER sessions. This issue is simpler: threads don't even load their OWN prior messages.

The root cause appears to be that when OpenClaw creates a new thread session, it doesn't fetch the thread history from Slack before initializing the session transcript.

Possible fix

When initializing a thread session for the first time:

1. Check if thread_ts exists in the routing context
2. If yes, fetch thread history from Slack API (conversations.replies with ts=thread_ts)
3. Load those messages into the new session transcript before processing the current message

This would ensure thread sessions start with complete context rather than a blank slate.