fix(slack): preserve thread context for proactive replies

Capture Slack thread metadata for cron and message-tool deliveries so replies stay in the originating thread, and hydrate first thread mentions with recent Slack context.

Made-with: Cursor

tests/tools/test_message_tool.py

@@ -50,3 +50,38 @@ async def test_message_tool_marks_channel_delivery_only_when_enabled() -> None:
 
     assert sent[0].metadata == {}
     assert sent[1].metadata == {"_record_channel_delivery": True}
+
+
+@pytest.mark.asyncio
+async def test_message_tool_inherits_metadata_for_same_target() -> None:
+    sent: list[OutboundMessage] = []
+
+    async def _send(msg: OutboundMessage) -> None:
+        sent.append(msg)
+
+    tool = MessageTool(send_callback=_send)
+    slack_meta = {"slack": {"thread_ts": "111.222", "channel_type": "channel"}}
+    tool.set_context("slack", "C123", metadata=slack_meta)
+
+    await tool.execute(content="thread reply")
+
+    assert sent[0].metadata == slack_meta
+
+
+@pytest.mark.asyncio
+async def test_message_tool_does_not_inherit_metadata_for_cross_target() -> None:
+    sent: list[OutboundMessage] = []
+
+    async def _send(msg: OutboundMessage) -> None:
+        sent.append(msg)
+
+    tool = MessageTool(send_callback=_send)
+    tool.set_context(
+        "slack",
+        "C123",
+        metadata={"slack": {"thread_ts": "111.222", "channel_type": "channel"}},
+    )
+
+    await tool.execute(content="channel reply", channel="slack", chat_id="C999")
+
+    assert sent[0].metadata == {}