Add Guardian V2 extension scaffold (#38336)

## What changed

- Add the `codex-guardian-v2` crate with an extension install entry point that does not register contributors yet.
- Register the crate in the Cargo workspace and add its Bazel target.

GitOrigin-RevId: 7c0c9479c6722d65f0f0dad9b0fd4a756e4649b3

codex-rs/ext/guardian-v2/src/lib.rs

@@ -0,0 +1,4 @@
+use codex_extension_api::ExtensionRegistryBuilder;
+
+/// Installs the Guardian V2 extension without registering contributors yet.
+pub fn install<C: Sync>(_registry: &mut ExtensionRegistryBuilder<C>) {}