execpolicy helpers (#7032)

this PR 
- adds a helper function to amend `.codexpolicy` files with new prefix
rules
- adds a utility to `Policy` allowing prefix rules to be added to
existing `Policy` structs

both additions will be helpful as we thread codexpolicy into the TUI
workflow

codex-rs/Cargo.lock

@@ -1283,6 +1283,7 @@ dependencies = [
  "serde_json",
  "shlex",
  "starlark",
+ "tempfile",
  "thiserror 2.0.17",
 ]