Exact/full match instead `prefix_rule`.

Question

### What variant of Codex are you using?

cli

### What feature would you like to see?

Having a way to not have codex match only the start of the list.

### Example
I want to explicitly allow
```shell
rm -rf build/
```
because removing that build folder is something it needs to do all the time.

However with a `prefix_rule(…)`  this now also allows
```shell
rm -rf build/ --no-preserve-root /
```

having an `exact_rule(…)` or `rule(…)` would help against such attacks/mistakes.

### Additional i