Add allowed_handles config to filter which users bot responds to

When bot.allowed_handles is configured, only mentions from those
handles will be processed. Empty list means respond to all (default).

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[e-mail]>

bsky.py

···
             'mention_preview': mention_text[:100] if mention_text else ''
         })
 
+        # Check if handle is in allowed list (if configured)
+        allowed_handles = get_config().get('bot.allowed_handles', [])
+        if allowed_handles and author_handle not in allowed_handles:
+            logger.info(f"[{correlation_id}] Skipping mention from @{author_handle} (not in allowed_handles)")
+            return True  # Remove from queue
+
         # Retrieve the entire thread associated with the mention
         try:
             thread = atproto_client.app.bsky.feed.get_post_thread({