This is in our style guide, we can cheaply enforce it, let's do it.
```
$ pre-commit
check-case-conflicts.....................................................Passed
check-executables-have-shebangs..........................................Passed
check-headers............................................................Failed
- hook id: check-headers
- exit code: 1
Missing pattern @file in file src/libexpr/value.hh
We found some header files that don't conform to the style guide.
The Lix style guide requests that header files:
- Begin with `#pragma once` so they only get parsed once
- Contain a doxygen comment (`/**` or `///`) containing `@file`, for
example, `///@file`, which will make doxygen generate docs for them.
When adding that, consider also adding a `@brief` with a sentence
explaining what the header is for.
For more details: https://wiki.lix.systems/link/3#bkmrk-header-files
check-merge-conflicts....................................................Passed
check-shebang-scripts-are-executable.....................................Passed
check-symlinks.......................................(no files to check)Skipped
end-of-file-fixer........................................................Passed
mixed-line-endings.......................................................Passed
no-commit-to-branch......................................................Passed
release-notes........................................(no files to check)Skipped
treefmt..................................................................Passed
trim-trailing-whitespace.................................................Passed
```
Fixes: #233
Change-Id: I77150b9298c844ffedd0f85cc5250ae9208502e3
