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>

.env.example

@@ -2,6 +2,7 @@
 DISCORD_TOKEN=your_discord_bot_token_here
 CHANNEL_ID=your_channel_id_here,optional_second_channel_id
 STARTUP_ANNOUNCEMENT=
+STARTUP_ANNOUNCEMENT_CHANNEL_ID=
 
 # Bot Configuration
 COMMAND_PREFIX=!