Refactor execpolicy fallback evaluation (#7544)

## Refactor of the `execpolicy` crate

To illustrate why we need this refactor, consider an agent attempting to
run `apple | rm -rf ./`. Suppose `apple` is allowed by `execpolicy`.
Before this PR, `execpolicy` would consider `apple` and `pear` and only
render one rule match: `Allow`. We would skip any heuristics checks on
`rm -rf ./` and immediately approve `apple | rm -rf ./` to run.

To fix this, we now thread a `fallback` evaluation function into
`execpolicy` that runs when no `execpolicy` rules match a given command.
In our example, we would run `fallback` on `rm -rf ./` and prevent
`apple | rm -rf ./` from being run without approval.

codex-rs/core/src/apply_patch.rs

@@ -71,7 +71,7 @@ pub(crate) async fn apply_patch(
                 .await;
             match rx_approve.await.unwrap_or_default() {
                 ReviewDecision::Approved
-                | ReviewDecision::ApprovedAllowPrefix { .. }
+                | ReviewDecision::ApprovedExecpolicyAmendment { .. }
                 | ReviewDecision::ApprovedForSession => {
                     InternalApplyPatchInvocation::DelegateToExec(ApplyPatchExec {
                         action,