Add multi-channel support for stock updates

Bot now supports sending updates to multiple Discord channels simultaneously.

Changes:
- CHANNEL_ID accepts comma-separated list of channel IDs
- Config parses and validates multiple channel IDs
- Hourly and market close updates sent to all configured channels
- Update .env.example to show multi-channel format
- Document multi-channel configuration in README

Example: CHANNEL_ID=1442203998932304035,9876543210123456789

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

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

bot.py

@@ -36,7 +36,7 @@ class StockBot(commands.Bot):
             logger.info("Using Yahoo Finance API for stock data")
 
         self.scheduler = AsyncIOScheduler(timezone=pytz.timezone('America/New_York'))
-        self.target_channel_id = int(Config.CHANNEL_ID)
+        self.target_channel_ids = [int(id) for id in Config.CHANNEL_IDS]
         self.primary_ticker = Config.PRIMARY_TICKER
 
     async def setup_hook(self):
@@ -67,14 +67,15 @@ class StockBot(commands.Bot):
         """Called when bot successfully connects to Discord."""
         logger.info(f'Bot connected as {self.user.name} (ID: {self.user.id})')
         logger.info(f'Monitoring ticker: {self.primary_ticker}')
-        logger.info(f'Target channel ID: {self.target_channel_id}')
+        logger.info(f'Target channel IDs: {self.target_channel_ids}')
 
-        # Verify channel exists
-        channel = self.get_channel(self.target_channel_id)
-        if channel:
-            logger.info(f'Target channel found: #{channel.name}')
-        else:
-            logger.error(f'Could not find channel with ID {self.target_channel_id}')
+        # Verify channels exist
+        for channel_id in self.target_channel_ids:
+            channel = self.get_channel(channel_id)
+            if channel:
+                logger.info(f'Target channel found: #{channel.name} (ID: {channel_id})')
+            else:
+                logger.error(f'Could not find channel with ID {channel_id}')
 
     async def send_hourly_update(self):
         """Send hourly stock price update if market is open."""
@@ -83,7 +84,8 @@ class StockBot(commands.Bot):
             return
 
         logger.info(f"Sending hourly update for {self.primary_ticker}")
-        await self.send_stock_update(self.primary_ticker, self.target_channel_id)
+        for channel_id in self.target_channel_ids:
+            await self.send_stock_update(self.primary_ticker, channel_id)
 
     async def send_market_close_update(self):
         """Send stock price update at market close on trading days."""
@@ -92,7 +94,8 @@ class StockBot(commands.Bot):
             return
 
         logger.info(f"Sending market close update for {self.primary_ticker}")
-        await self.send_stock_update(self.primary_ticker, self.target_channel_id)
+        for channel_id in self.target_channel_ids:
+            await self.send_stock_update(self.primary_ticker, channel_id)
 
     async def send_stock_update(self, ticker: str, channel_id: int):
         """