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/codex_delegate.rs

@@ -281,7 +281,7 @@ async fn handle_exec_approval(
         event.cwd,
         event.reason,
         event.risk,
-        event.allow_prefix,
+        event.proposed_execpolicy_amendment,
     );
     let decision = await_approval_with_cancel(
         approval_fut,