`--exclude` option has no effect

As titled:

```
$ deadnix --exclude packages/shells/cod/default.nix -- \
| grep packages --context=3

70 │ (final: prev: {
│ ╰─── Unused lambda argument: final
Warning: Unused declarations were found.
╭─[./packages/shells/cod/default.nix:2:3]
2 │ stdenv,
│ ╰─── Unused lambda pattern: stdenv
Warning: Unused declarations were found.
```

```
$ deadnix --exclude packages/shells/cod/default.nix -- packages/shells/cod/default.nix \
| grep packages --context=3

Warning: Unused declarations were found.
╭─[packages/shells/cod/default.nix:2:3]
2 │ stdenv,
│ ╰─── Unused lambda pattern: stdenv
```

Edit: This is v1.2.0 from nixos-unstable. I figured that'd be the most recent, but it looks like there's also v1.2.1. However, it makes no difference:

```
$ nix run github:astro/deadnix -- --exclude packages/shells/cod/default.nix -- packages/shells/cod/default.nix
Warning: Unused declarations were found.
╭─[packages/shells/cod/default.nix:2:3]
2 │ stdenv,
│ ╰─── Unused lambda pattern: stdenv
```

Start with the deadnix CLI entry point and the handling of the --exclude argument, then reproduce the commands using packages/shells/cod/default.nix. Done means excluding that path suppresses its unused-declaration warnings in both directory scans and explicit-file input.

Written by the indexing model from the issue text.