refactor: standardize config path and add CLI entry point

- Move EnvironmentFile from /opt/aitbc/.env to /etc/aitbc/blockchain.env in all systemd services
- Add main() entry point function to CLI for package installation compatibility
- Update aitbc-blockchain-node.service, aitbc-blockchain-node-dev.service
- Update aitbc-blockchain-rpc.service
- Update aitbc-blockchain-sync.service, aitbc-blockchain-sync-dev.service

This aligns with Linux filesystem hierarchy standards where /etc/ is the proper

cli/core/main.py

@@ -247,5 +247,10 @@ def version(ctx):
     output(f"AITBC CLI version {__version__}", ctx.obj['output_format'])
 
 
+def main():
+    """Main entry point for AITBC CLI"""
+    return cli()
+
+
 if __name__ == "__main__":
     cli()