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>

config.py

@@ -13,6 +13,7 @@ class Config:
     CHANNEL_IDS = [id.strip() for id in os.getenv('CHANNEL_ID', '').split(',') if id.strip()]
     COMMAND_PREFIX = os.getenv('COMMAND_PREFIX', '!')
     STARTUP_ANNOUNCEMENT = os.getenv('STARTUP_ANNOUNCEMENT', '')
+    STARTUP_ANNOUNCEMENT_CHANNEL_ID = os.getenv('STARTUP_ANNOUNCEMENT_CHANNEL_ID', '')
 
     # Stock configuration
     PRIMARY_TICKER = os.getenv('PRIMARY_TICKER', 'PYPL')