build(operator): declare standalone [workspace] so CI cargo check works

The ci.yml `operator` job runs `cargo check` with working-directory
operator, but the crate is not listed in the repo-root workspace
(members: openab-core, openab-gateway). Cargo then walks up to the root
Cargo.toml and errors ("believes it's in a workspace when it's not").
This job only runs when operator/** changes, so it was latent until now.

Adding an empty [workspace] table marks operator as its own workspace
root, matching how it's built and released standalone (own Cargo.lock).

Author: chaodu-agent

operator/Cargo.toml

@@ -4,6 +4,11 @@ version = "0.1.0"
 edition = "2021"
 description = "CLI provisioner for OAB agents on ECS"
 
+# Standalone crate — not part of the repo-root [workspace]. Declaring an empty
+# workspace table stops cargo from attaching this crate to the root workspace
+# (which lists only crates/openab-core and crates/openab-gateway).
+[workspace]
+
 [[bin]]
 name = "oabctl"
 path = "src/main.rs"