fix(gateway): add platform-specific notes to session context prompt (#1184)

Tell the agent what it CANNOT do on Slack and Discord — no searching
channel history, no pinning messages, no managing channels/roles.
Prevents the agent from hallucinating capabilities it doesn't have
and promising actions it can't deliver.

Addresses user feedback: agent says 'I'll search your Slack history'
then goes silent because no Slack-specific tools exist.

tests/gateway/test_session.py

@@ -182,7 +182,7 @@ class TestBuildSessionContextPrompt:
             platforms={
                 Platform.DISCORD: PlatformConfig(
                     enabled=True,
-                    token="fake-discord-token",
+                    token="fake-d...oken",
                 ),
             },
         )
@@ -197,6 +197,27 @@ class TestBuildSessionContextPrompt:
         prompt = build_session_context_prompt(ctx)
 
         assert "Discord" in prompt
+        assert "cannot search" in prompt.lower() or "do not have access" in prompt.lower()
+
+    def test_slack_prompt_includes_platform_notes(self):
+        config = GatewayConfig(
+            platforms={
+                Platform.SLACK: PlatformConfig(enabled=True, token="fake"),
+            },
+        )
+        source = SessionSource(
+            platform=Platform.SLACK,
+            chat_id="C123",
+            chat_name="general",
+            chat_type="group",
+            user_name="bob",
+        )
+        ctx = build_session_context(source, config)
+        prompt = build_session_context_prompt(ctx)
+
+        assert "Slack" in prompt
+        assert "cannot search" in prompt.lower()
+        assert "pin" in prompt.lower()
 
     def test_discord_prompt_with_channel_topic(self):
         """Channel topic should appear in the session context prompt."""