chore: goal ext skeleton (#23288)

Skeleton of `/goal` in extension
Lot's of follow-ups coming

codex-rs/Cargo.lock

@@ -2946,6 +2946,18 @@ dependencies = [
  "walkdir",
 ]
 
+[[package]]
+name = "codex-goal-extension"
+version = "0.0.0"
+dependencies = [
+ "async-trait",
+ "codex-extension-api",
+ "codex-protocol",
+ "codex-tools",
+ "serde",
+ "serde_json",
+]
+
 [[package]]
 name = "codex-guardian"
 version = "0.0.0"