Completions for sub-directory Justfiles are missing/broken.

I have a project like,

```
.
├── app
│   └── Justfile # recipe_1
├── db
│   └── Justfile # recipe_2
└── Justfile # global_recipe
```

Where I can run [recipes in other directories](https://just.systems/man/en/invoking-justfiles-in-other-directories.html)

```fish
# Global recipes
just global_recipe

# But also
just app/recipe_1
just db/recipe_2
```

However, at least in fish I only get completions for `just `, but not for `just app/` nor `just db/`.

---

I'm using just `v1.57.0`

Start by reproducing the sample directory layout and testing `just app/` and `just db/` in fish, then inspect just's fish completion support. Done means completions list recipes from sub-directory Justfiles while preserving the existing completions for top-level recipes.

Written by the indexing model from the issue text.