feat(slack): add default done reaction on completion

tests/test_slack_channel.py

@@ -12,6 +12,8 @@ class _FakeAsyncWebClient:
     def __init__(self) -> None:
         self.chat_post_calls: list[dict[str, object | None]] = []
         self.file_upload_calls: list[dict[str, object | None]] = []
+        self.reactions_add_calls: list[dict[str, object | None]] = []
+        self.reactions_remove_calls: list[dict[str, object | None]] = []
 
     async def chat_postMessage(
         self,
@@ -43,6 +45,36 @@ class _FakeAsyncWebClient:
             }
         )
 
+    async def reactions_add(
+        self,
+        *,
+        channel: str,
+        name: str,
+        timestamp: str,
+    ) -> None:
+        self.reactions_add_calls.append(
+            {
+                "channel": channel,
+                "name": name,
+                "timestamp": timestamp,
+            }
+        )
+
+    async def reactions_remove(
+        self,
+        *,
+        channel: str,
+        name: str,
+        timestamp: str,
+    ) -> None:
+        self.reactions_remove_calls.append(
+            {
+                "channel": channel,
+                "name": name,
+                "timestamp": timestamp,
+            }
+        )
+
 
 @pytest.mark.asyncio
 async def test_send_uses_thread_for_channel_messages() -> None:
@@ -88,3 +120,28 @@ async def test_send_omits_thread_for_dm_messages() -> None:
     assert fake_web.chat_post_calls[0]["thread_ts"] is None
     assert len(fake_web.file_upload_calls) == 1
     assert fake_web.file_upload_calls[0]["thread_ts"] is None
+
+
+@pytest.mark.asyncio
+async def test_send_updates_reaction_when_final_response_sent() -> None:
+    channel = SlackChannel(SlackConfig(enabled=True, react_emoji="eyes"), MessageBus())
+    fake_web = _FakeAsyncWebClient()
+    channel._web_client = fake_web
+
+    await channel.send(
+        OutboundMessage(
+            channel="slack",
+            chat_id="C123",
+            content="done",
+            metadata={
+                "slack": {"event": {"ts": "1700000000.000100"}, "channel_type": "channel"},
+            },
+        )
+    )
+
+    assert fake_web.reactions_remove_calls == [
+        {"channel": "C123", "name": "eyes", "timestamp": "1700000000.000100"}
+    ]
+    assert fake_web.reactions_add_calls == [
+        {"channel": "C123", "name": "white_check_mark", "timestamp": "1700000000.000100"}
+    ]