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>

README.md

@@ -159,11 +159,16 @@ Environment variables in `.env`:
 | Variable | Description | Default |
 |----------|-------------|---------|
 | `DISCORD_TOKEN` | Your Discord bot token | Required |
-| `CHANNEL_ID` | Discord channel ID for updates | Required |
+| `CHANNEL_ID` | Discord channel ID(s) for updates (comma-separated for multiple) | Required |
 | `COMMAND_PREFIX` | Command prefix for bot | `!` |
 | `PRIMARY_TICKER` | Stock ticker for hourly updates | `PYPL` |
 | `UPDATE_INTERVAL_HOURS` | Hours between updates | `1` |
 
+**Multi-Channel Support:** To send updates to multiple channels, provide comma-separated channel IDs:
+```
+CHANNEL_ID=1442203998932304035,9876543210123456789
+```
+
 ## Switching Stock API Providers
 
 The bot is designed with an abstract API layer. To switch providers: