fix: _get_channel fetches existing channels before creating (prevents duplicates)

bot.py

@@ -199,11 +199,27 @@ class GravityBot(commands.Bot):
             return self.project_channels[project_name]
 
         async with self._channel_lock:
-            # Double-check
+            # Double-check after lock
             if project_name in self.project_channels:
                 return self.project_channels[project_name]
 
             channel_name = self._make_channel_name(project_name)
+
+            # Search existing channels FIRST (prevents duplicates)
+            try:
+                all_channels = await self.guild.fetch_channels()
+                for ch in all_channels:
+                    if (isinstance(ch, discord.TextChannel)
+                            and ch.name == channel_name
+                            and ch.category_id == self.session_category.id):
+                        self.project_channels[project_name] = ch
+                        self.channel_to_project[ch.id] = project_name
+                        logger.info(f"Found existing channel: #{channel_name}")
+                        return ch
+            except Exception as e:
+                logger.warning(f"fetch_channels failed: {e}")
+
+            # No existing channel — create new
             try:
                 ch = await self.guild.create_text_channel(
                     name=channel_name,