fix: Slack thread handling — progress messages, responses, and session isolation

Three bugs fixed in the Slack adapter:

1. Tool progress messages leaked to main channel instead of thread.
   Root cause: metadata key mismatch — gateway uses 'thread_id' but
   Slack adapter checked for 'thread_ts'. Added _resolve_thread_ts()
   helper that checks both keys with correct precedence.

2. Bot responses could escape threads for replies.
   Root cause: reply_to was set to the child message's ts, but Slack
   API needs the parent message's ts for thread_ts. Now metadata
   thread_id (always the parent ts) takes priority over reply_to.

3. All Slack DMs shared one session key ('agent:main:slack:dm'),
   so a long-running task blocked all other DM conversations.
   Fix: DMs with thread_id now get per-thread session keys. Top-level
   DMs still share one session for conversation continuity.

Additional fix: All Slack media methods (send_image, send_voice,
send_video, send_document, send_image_file) now accept metadata
parameter for thread routing. Previously they only accepted reply_to,
which caused media to silently fail to post in threads.

Session key behavior after this change:
- Slack channel @mention: creates thread, thread = session
- Slack thread reply: stays in thread, same session
- Slack DM (top-level): one continuous session
- Slack DM (threaded): per-thread session
- Other platforms: unchanged

gateway/session.py

@@ -299,10 +299,21 @@ def build_session_key(source: SessionSource) -> str:
     """Build a deterministic session key from a message source.
 
     This is the single source of truth for session key construction.
-    WhatsApp DMs include chat_id (multi-user), other DMs do not (single owner).
+
+    DM rules:
+      - WhatsApp DMs include chat_id (multi-user support).
+      - Other DMs include thread_id when present (e.g. Slack threaded DMs),
+        so each DM thread gets its own session while top-level DMs share one.
+      - Without thread_id or chat_id, all DMs share a single session.
+
+    Group/channel rules:
+      - thread_id differentiates threads within a channel.
+      - Without thread_id, all messages in a channel share one session.
     """
     platform = source.platform.value
     if source.chat_type == "dm":
+        if source.thread_id:
+            return f"agent:main:{platform}:dm:{source.thread_id}"
         if platform == "whatsapp" and source.chat_id:
             return f"agent:main:{platform}:dm:{source.chat_id}"
         return f"agent:main:{platform}:dm"