feat(gateway): include Discord channel topic in session context

Fixes #163

- Add chat_topic field to SessionSource dataclass
- Update to_dict/from_dict for serialization support
- Add chat_topic parameter to build_source helper
- Extract channel.topic in Discord adapter for messages and slash commands
- Display Channel Topic in system prompt when available
- Normalize empty topics to None

gateway/platforms/base.py

@@ -736,9 +736,13 @@ class BasePlatformAdapter(ABC):
         chat_type: str = "dm",
         user_id: Optional[str] = None,
         user_name: Optional[str] = None,
-        thread_id: Optional[str] = None
+        thread_id: Optional[str] = None,
+        chat_topic: Optional[str] = None,
     ) -> SessionSource:
         """Helper to build a SessionSource for this platform."""
+        # Normalize empty topic to None
+        if chat_topic is not None and not chat_topic.strip():
+            chat_topic = None
         return SessionSource(
             platform=self.platform,
             chat_id=str(chat_id),
@@ -747,6 +751,7 @@ class BasePlatformAdapter(ABC):
             user_id=str(user_id) if user_id else None,
             user_name=user_name,
             thread_id=str(thread_id) if thread_id else None,
+            chat_topic=chat_topic.strip() if chat_topic else None,
         )
     
     @abstractmethod

gateway/platforms/discord.py

@@ -542,6 +542,9 @@ class DiscordAdapter(BasePlatformAdapter):
             chat_name = interaction.channel.name
             if hasattr(interaction.channel, "guild") and interaction.channel.guild:
                 chat_name = f"{interaction.channel.guild.name} / #{chat_name}"
+        
+        # Get channel topic (if available)
+        chat_topic = getattr(interaction.channel, "topic", None)
 
         source = self.build_source(
             chat_id=str(interaction.channel_id),
@@ -549,6 +552,7 @@ class DiscordAdapter(BasePlatformAdapter):
             chat_type=chat_type,
             user_id=str(interaction.user.id),
             user_name=interaction.user.display_name,
+            chat_topic=chat_topic,
         )
 
         msg_type = MessageType.COMMAND if text.startswith("/") else MessageType.TEXT
@@ -661,6 +665,9 @@ class DiscordAdapter(BasePlatformAdapter):
         if isinstance(message.channel, discord.Thread):
             thread_id = str(message.channel.id)
         
+        # Get channel topic (if available - TextChannels have topics, DMs/threads don't)
+        chat_topic = getattr(message.channel, "topic", None)
+        
         # Build source
         source = self.build_source(
             chat_id=str(message.channel.id),
@@ -669,6 +676,7 @@ class DiscordAdapter(BasePlatformAdapter):
             user_id=str(message.author.id),
             user_name=message.author.display_name,
             thread_id=thread_id,
+            chat_topic=chat_topic,
         )
         
         # Build media URLs -- download image attachments to local cache so the

gateway/session.py

@@ -44,6 +44,7 @@ class SessionSource:
     user_id: Optional[str] = None
     user_name: Optional[str] = None
     thread_id: Optional[str] = None  # For forum topics, Discord threads, etc.
+    chat_topic: Optional[str] = None  # Channel topic/description (Discord, Slack)
     
     @property
     def description(self) -> str:
@@ -75,6 +76,7 @@ class SessionSource:
             "user_id": self.user_id,
             "user_name": self.user_name,
             "thread_id": self.thread_id,
+            "chat_topic": self.chat_topic,
         }
     
     @classmethod
@@ -87,6 +89,7 @@ class SessionSource:
             user_id=data.get("user_id"),
             user_name=data.get("user_name"),
             thread_id=data.get("thread_id"),
+            chat_topic=data.get("chat_topic"),
         )
     
     @classmethod
@@ -154,6 +157,10 @@ def build_session_context_prompt(context: SessionContext) -> str:
         lines.append(f"**Source:** {platform_name} (the machine running this agent)")
     else:
         lines.append(f"**Source:** {platform_name} ({context.source.description})")
+    
+    # Channel topic (if available - provides context about the channel's purpose)
+    if context.source.chat_topic:
+        lines.append(f"**Channel Topic:** {context.source.chat_topic}")
 
     # User identity (especially useful for WhatsApp where multiple people DM)
     if context.source.user_name: