feat(discord): inherit forum channel topic in thread sessions

ORIGINAL INCIDENT:
Discord forum descriptions (the topic field on ForumChannel) were invisible
to the agent. When a user set project instructions in a forum's description
(e.g. tool-evaluations), threads created in that forum had no Channel Topic
in their session context. Discovered while evaluating per-forum auto-context
injection for web-tap-terminal development threads.

ISSUE IN THE CODE:
In gateway/platforms/discord.py, all three session entry points
(_handle_message, _build_slash_event, _dispatch_thread_session) read
chat_topic via getattr(channel, 'topic', None). Discord Thread objects
don't carry a topic — only the parent ForumChannel does. So chat_topic
was always None for forum threads, and the Channel Topic line was never
injected into build_session_context_prompt output. The infrastructure to
handle this was already in place — _is_forum_parent() detects forum
channels, _format_thread_chat_name() traverses to the parent, and
build_session_context_prompt() renders Channel Topic when present. The
forum parent was being identified; its topic just wasn't being read.

HOW THIS COMMIT FIXES IT:
Adds _get_effective_topic(channel, is_thread) helper that reads
channel.topic first, then falls back to the parent forum's topic when
the channel is a thread inside a forum. All three session entry points
now call this helper instead of inlining getattr(channel, 'topic', None).
Existing tests pass unchanged.

Co-authored-by: dhabibi <9087935+dhabibi@users.noreply.github.com>

gateway/platforms/discord.py

@@ -1767,8 +1767,9 @@ class DiscordAdapter(BasePlatformAdapter):
             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)
+        # Get channel topic (if available).
+        # For forum threads, inherit the parent forum's topic.
+        chat_topic = self._get_effective_topic(interaction.channel, is_thread=is_thread)
 
         source = self.build_source(
             chat_id=str(interaction.channel_id),
@@ -1842,6 +1843,10 @@ class DiscordAdapter(BasePlatformAdapter):
 
         chat_name = f"{guild_name} / {thread_name}" if guild_name else thread_name
 
+        # Inherit forum topic when the thread was created inside a forum channel.
+        _chan = getattr(interaction, "channel", None)
+        chat_topic = self._get_effective_topic(_chan, is_thread=True) if _chan else None
+
         source = self.build_source(
             chat_id=thread_id,
             chat_name=chat_name,
@@ -1849,6 +1854,7 @@ class DiscordAdapter(BasePlatformAdapter):
             user_id=str(interaction.user.id),
             user_name=interaction.user.display_name,
             thread_id=thread_id,
+            chat_topic=chat_topic,
         )
 
         event = MessageEvent(
@@ -2134,6 +2140,15 @@ class DiscordAdapter(BasePlatformAdapter):
                 return True
         return False
 
+    def _get_effective_topic(self, channel: Any, is_thread: bool = False) -> Optional[str]:
+        """Return the channel topic, falling back to the parent forum's topic for forum threads."""
+        topic = getattr(channel, "topic", None)
+        if not topic and is_thread:
+            parent = getattr(channel, "parent", None)
+            if parent and self._is_forum_parent(parent):
+                topic = getattr(parent, "topic", None)
+        return topic
+
     def _format_thread_chat_name(self, thread: Any) -> str:
         """Build a readable chat name for thread-like Discord channels, including forum context when available."""
         thread_name = getattr(thread, "name", None) or str(getattr(thread, "id", "thread"))
@@ -2301,8 +2316,10 @@ class DiscordAdapter(BasePlatformAdapter):
             if hasattr(message.channel, "guild") and message.channel.guild:
                 chat_name = f"{message.channel.guild.name} / #{chat_name}"
 
-        # Get channel topic (if available - TextChannels have topics, DMs/threads don't)
-        chat_topic = getattr(message.channel, "topic", None)
+        # Get channel topic (if available - TextChannels have topics, DMs/threads don't).
+        # For threads whose parent is a forum channel, inherit the parent's topic
+        # so forum descriptions (e.g. project instructions) appear in the session context.
+        chat_topic = self._get_effective_topic(message.channel, is_thread=is_thread)
 
         # Build source
         source = self.build_source(