Add optional startup announcement feature

Bot can now send a one-time announcement to all configured channels when it starts.

Configuration:
- Set STARTUP_ANNOUNCEMENT in .env with your message
- Leave empty to disable announcements
- Prevents duplicate announcements on reconnects

Usage example:
STARTUP_ANNOUNCEMENT=Bot updated! New features: colored prices and charts

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

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

bot.py

@@ -38,6 +38,7 @@ class StockBot(commands.Bot):
         self.scheduler = AsyncIOScheduler(timezone=pytz.timezone('America/New_York'))
         self.target_channel_ids = [int(id) for id in Config.CHANNEL_IDS]
         self.primary_ticker = Config.PRIMARY_TICKER
+        self.startup_announced = False
 
     async def setup_hook(self):
         """Initialize the bot and set up scheduled tasks."""
@@ -77,6 +78,11 @@ class StockBot(commands.Bot):
             else:
                 logger.error(f'Could not find channel with ID {channel_id}')
 
+        # Send startup announcement if configured and not already sent
+        if Config.STARTUP_ANNOUNCEMENT and not self.startup_announced:
+            await self.send_startup_announcement()
+            self.startup_announced = True
+
     async def send_hourly_update(self):
         """Send hourly stock price update if market is open."""
         if not MarketHours.is_market_open():
@@ -97,6 +103,23 @@ class StockBot(commands.Bot):
         for channel_id in self.target_channel_ids:
             await self.send_stock_update(self.primary_ticker, channel_id)
 
+    async def send_startup_announcement(self):
+        """Send startup announcement to configured channels."""
+        embed = discord.Embed(
+            title="🤖 Bot Update",
+            description=Config.STARTUP_ANNOUNCEMENT,
+            color=discord.Color.blue(),
+            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}")
+
     async def send_stock_update(self, ticker: str, channel_id: int):
         """
         Fetch stock data and send formatted embed to specified channel.