fix(slack): preserve thread-parent context when cron/bot posted the parent

The Slack thread-context fetcher used to drop every message with a
bot_id, which silently erased the thread parent whenever a cron job (or
any other bot) had posted it. As a result, replies to a cron-posted
summary lost all context and the agent answered as if from a blank
thread.

Changes:

1. gateway/platforms/slack.py::_fetch_thread_context
   - Keep the thread parent even when it was posted by a bot
     (e.g. cron summaries, third-party integrations).
   - Only skip *our own* prior bot replies to avoid circular context,
     matching the per-workspace bot user id via _team_bot_user_ids so
     multi-workspace deployments stay correct.
   - Keep non-self bot children (useful third-party context).

2. gateway/platforms/slack.py::_handle_slack_message
   - Populate MessageEvent.reply_to_text for thread replies (parity
     with Telegram/Discord/Feishu/WeCom). gateway.run uses this field
     to inject a [Replying to: "..."] prefix when the parent is not
     already in the session history, which is exactly the scenario
     triggered by cron-generated thread parents.
   - New helper _fetch_thread_parent_text reuses the existing thread-
     context cache (and its 60s TTL) to avoid duplicate
     conversations.replies calls; falls back to a cheap limit=1 fetch
     when the cache is cold.

Tests:

- Updated TestSlackThreadContext::test_skips_bot_messages to reflect
  the new behaviour (self-bot child dropped, third-party bot kept).
- Added:
    * test_fetch_thread_context_includes_bot_parent
    * test_fetch_thread_context_excludes_self_bot_replies
    * test_fetch_thread_context_multi_workspace
    * test_fetch_thread_context_current_ts_excluded (regression guard)
    * test_fetch_thread_parent_text_from_cache
    * test_slack_reply_to_text_set_on_thread_reply
    * test_slack_reply_to_text_none_for_top_level_message

Full Slack suite: 176 passed (was 169).

tests/gateway/test_slack.py

@@ -2011,3 +2011,76 @@ class TestProgressMessageThread:
             "so each @mention starts its own thread"
         )
         assert msg_event.message_id == "2000000000.000001"
+
+
+class TestSlackReplyToText:
+    """Ensure MessageEvent.reply_to_text is populated on thread replies so
+    gateway.run can inject a ``[Replying to: "..."]`` prefix (parity with
+    Telegram/Discord/Feishu/WeCom)."""
+
+    @pytest.mark.asyncio
+    async def test_slack_reply_to_text_set_on_thread_reply(self, adapter):
+        """When a thread reply arrives and the parent was posted by a bot
+        (e.g. cron summary), reply_to_text must carry the parent's text."""
+        adapter._channel_team = {}  # primary workspace only
+        adapter._team_bot_user_ids = {}
+
+        # Mock conversations_replies to return a bot-posted parent
+        adapter._app.client.conversations_replies = AsyncMock(return_value={
+            "messages": [
+                {
+                    "ts": "1000.0",
+                    "bot_id": "B_CRON",
+                    "text": "メール要約: 新着メール3件あります",
+                },
+                {"ts": "1000.5", "user": "U_USER", "text": "詳細を教えて"},
+            ]
+        })
+
+        # Use a DM so mention-gating doesn't short-circuit the handler.
+        event = {
+            "text": "詳細を教えて",
+            "user": "U_USER",
+            "channel": "D123",
+            "channel_type": "im",
+            "ts": "1000.5",
+            "thread_ts": "1000.0",  # thread reply
+        }
+
+        with patch.object(
+            adapter, "_resolve_user_name", new=AsyncMock(return_value="Alice")
+        ):
+            await adapter._handle_slack_message(event)
+
+        assert adapter.handle_message.call_args is not None, (
+            "handle_message must be invoked for thread-reply DM"
+        )
+        msg_event = adapter.handle_message.call_args[0][0]
+        assert msg_event.reply_to_message_id == "1000.0"
+        # The critical assertion: parent text is exposed as reply_to_text so the
+        # gateway can inject it when not already in the session history.
+        assert msg_event.reply_to_text is not None
+        assert "メール要約" in msg_event.reply_to_text
+
+    @pytest.mark.asyncio
+    async def test_slack_reply_to_text_none_for_top_level_message(self, adapter):
+        """Top-level messages (no thread_ts) must not set reply_to_text."""
+        event = {
+            "text": "hello",
+            "user": "U_USER",
+            "channel": "D123",
+            "channel_type": "im",
+            "ts": "1000.0",
+            # no thread_ts — top-level DM
+        }
+
+        with patch.object(
+            adapter, "_resolve_user_name", new=AsyncMock(return_value="Alice")
+        ):
+            await adapter._handle_slack_message(event)
+
+        assert adapter.handle_message.call_args is not None
+        msg_event = adapter.handle_message.call_args[0][0]
+        assert msg_event.reply_to_text is None
+        # Top-level message: reply_to_message_id must be falsy (None or empty).
+        assert not msg_event.reply_to_message_id