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/BUILD.bazel

@@ -0,0 +1,6 @@
+load("//:defs.bzl", "codex_rust_crate")
+
+codex_rust_crate(
+    name = "guardian-v2",
+    crate_name = "codex_guardian_v2",
+)

codex-rs/ext/guardian-v2/Cargo.toml

@@ -0,0 +1,17 @@
+[package]
+edition.workspace = true
+license.workspace = true
+name = "codex-guardian-v2"
+version.workspace = true
+
+[lib]
+name = "codex_guardian_v2"
+path = "src/lib.rs"
+test = false
+doctest = false
+
+[lints]
+workspace = true
+
+[dependencies]
+codex-extension-api = { workspace = true }

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>) {}