New hook? uv-update-requirements

Why not have a hook that can export old school requirements.txt and update the legacy requirements.txt without interrupting git commit?

I wanted to achieve similar with uv-export hook, but it was not possible to do this.

```
- repo: local
hooks:
- id: update-requirements
name: Update requirements.txt
entry: bash -c 'uv pip freeze > requirements.txt' # EDITED OUT && git add requirements.txt #
language: system
types: [file]
files: \.(json|py|toml)
stages: [pre-commit]
```

Thought it'd be a legit addition.

Start by locating the existing uv-export hook and its pre-commit configuration. Done means a hook can export and update requirements.txt during pre-commit without interrupting the commit, including the generated-file handling illustrated in the issue.

Written by the indexing model from the issue text.