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>

README.md

@@ -160,6 +160,7 @@ Environment variables in `.env`:
 |----------|-------------|---------|
 | `DISCORD_TOKEN` | Your Discord bot token | Required |
 | `CHANNEL_ID` | Discord channel ID(s) for updates (comma-separated for multiple) | Required |
+| `STARTUP_ANNOUNCEMENT` | Optional message to send when bot starts (empty = no announcement) | `""` |
 | `COMMAND_PREFIX` | Command prefix for bot | `!` |
 | `PRIMARY_TICKER` | Stock ticker for hourly updates | `PYPL` |
 | `UPDATE_INTERVAL_HOURS` | Hours between updates | `1` |
@@ -169,6 +170,11 @@ Environment variables in `.env`:
 CHANNEL_ID=1442203998932304035,9876543210123456789
 ```
 
+**Startup Announcements:** Send a one-time message when the bot starts (useful for announcing updates):
+```
+STARTUP_ANNOUNCEMENT=Bot updated with new features: colored price changes and FinViz charts!
+```
+
 ## Switching Stock API Providers
 
 The bot is designed with an abstract API layer. To switch providers: