fix(slack): per-thread sessions for DMs by default

Each top-level Slack DM now gets its own Hermes session, matching the
per-thread behavior channels already have. Previously all top-level DM
messages shared one continuous session because thread_ts was None,
causing context to accumulate across unrelated conversations.

The behavior is controlled by platforms.slack.extra.dm_top_level_threads_as_sessions
in config.yaml (default: true). Set to false to restore legacy behavior.

Based on PR #10789 by helix4u. Changes from original:
- Default flipped to true (was opt-in, now opt-out)
- Removed env var fallback (config.yaml only per project policy)
- Tests updated to cover both default and opt-out paths

tests/gateway/test_slack.py

@@ -1678,11 +1678,11 @@ class TestProgressMessageThread:
         msg_event = captured_events[0]
         source = msg_event.source
 
-        # For a top-level DM: source.thread_id should remain None
-        # (session keying must not be affected)
-        assert source.thread_id is None, (
-            "source.thread_id must stay None for top-level DMs "
-            "so they share one continuous session"
+        # With default dm_top_level_threads_as_sessions=True, source.thread_id
+        # should equal the message ts so each DM thread gets its own session.
+        assert source.thread_id == "1234567890.000001", (
+            "source.thread_id must equal the message ts for top-level DMs "
+            "so each reply thread gets its own session"
         )
 
         # The message_id should be the event's ts — this is what the gateway
@@ -1707,6 +1707,34 @@ class TestProgressMessageThread:
             "ensuring progress messages land in the thread"
         )
 
+    @pytest.mark.asyncio
+    async def test_dm_toplevel_shares_session_when_disabled(self, adapter):
+        """Opting out restores legacy single-session-per-DM-channel behavior."""
+        adapter.config.extra["dm_top_level_threads_as_sessions"] = False
+
+        event = {
+            "channel": "D_DM",
+            "channel_type": "im",
+            "user": "U_USER",
+            "text": "Hello bot",
+            "ts": "1234567890.000001",
+        }
+
+        captured_events = []
+        adapter.handle_message = AsyncMock(side_effect=lambda e: captured_events.append(e))
+
+        with patch.object(adapter, "_resolve_user_name", new=AsyncMock(return_value="testuser")):
+            await adapter._handle_slack_message(event)
+
+        assert len(captured_events) == 1
+        msg_event = captured_events[0]
+        source = msg_event.source
+
+        assert source.thread_id is None, (
+            "source.thread_id must stay None when "
+            "dm_top_level_threads_as_sessions is disabled"
+        )
+
     @pytest.mark.asyncio
     async def test_channel_mention_progress_uses_thread_ts(self, adapter):
         """Progress messages for a channel @mention should go into the reply thread."""