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>

.env.example

@@ -1,6 +1,6 @@
 # Discord Configuration
 DISCORD_TOKEN=your_discord_bot_token_here
-CHANNEL_ID=your_channel_id_here
+CHANNEL_ID=your_channel_id_here,optional_second_channel_id
 
 # Bot Configuration
 COMMAND_PREFIX=!