Add dedicated channel for startup announcements

Add STARTUP_ANNOUNCEMENT_CHANNEL_ID to control where startup announcements are sent.

Changes:
- New config: STARTUP_ANNOUNCEMENT_CHANNEL_ID
- Announcements sent only to specified channel
- Falls back to first CHANNEL_ID if not set
- Prevents announcement spam in all stock update channels

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

bot.py

@@ -104,7 +104,7 @@ class StockBot(commands.Bot):
             await self.send_stock_update(self.primary_ticker, channel_id)
 
     async def send_startup_announcement(self):
-        """Send startup announcement to configured channels."""
+        """Send startup announcement to configured announcement channel."""
         embed = discord.Embed(
             title="🤖 Bot Update",
             description=Config.STARTUP_ANNOUNCEMENT,
@@ -112,13 +112,27 @@ class StockBot(commands.Bot):
             timestamp=datetime.now(pytz.timezone('America/New_York'))
         )
 
-        for channel_id in self.target_channel_ids:
-            channel = self.get_channel(channel_id)
-            if channel:
-                await channel.send(embed=embed)
-                logger.info(f"Sent startup announcement to channel {channel_id}")
-            else:
-                logger.error(f"Could not send announcement to channel {channel_id}")
+        # Use dedicated announcement channel if configured, otherwise use first target channel
+        announcement_channel_id = Config.STARTUP_ANNOUNCEMENT_CHANNEL_ID
+        if announcement_channel_id:
+            try:
+                channel_id = int(announcement_channel_id)
+            except ValueError:
+                logger.error(f"Invalid STARTUP_ANNOUNCEMENT_CHANNEL_ID: {announcement_channel_id}")
+                return
+        else:
+            channel_id = self.target_channel_ids[0] if self.target_channel_ids else None
+
+        if not channel_id:
+            logger.error("No channel configured for startup announcement")
+            return
+
+        channel = self.get_channel(channel_id)
+        if channel:
+            await channel.send(embed=embed)
+            logger.info(f"Sent startup announcement to channel {channel_id}")
+        else:
+            logger.error(f"Could not find announcement channel {channel_id}")
 
     async def send_stock_update(self, ticker: str, channel_id: int):
         """