Read Python version from .tool-versions (#996)

## Summary
- read the Python version from an explicitly selected `.tool-versions`
file
- preserve `python-version` and existing `UV_PYTHON` precedence
- add parser, input, and workflow coverage and update documentation and
bundled action artifacts

## Validation
- `npm run all`
- `actionlint .github/workflows/test.yml`
- `uvx zizmor .github/workflows/test.yml`

Closes #983

Refs: pi-session 019ff01a-544c-79f3-8f73-a00132af39f5

docs/advanced-version-configuration.md

@@ -85,7 +85,10 @@ You can use the `version-file` input to specify a file that contains the version
 This can either be a `pyproject.toml` or `uv.toml` file which defines a `required-version` or
 uv defined as a dependency in `pyproject.toml` or `requirements.txt`.
 
-[asdf](https://asdf-vm.com/) `.tool-versions` is also supported, but without the `ref` syntax.
+[asdf](https://asdf-vm.com/) `.tool-versions` is also supported for selecting uv. If neither
+`python-version` nor `UV_PYTHON` is set, the `python` entry from the selected file is also exported
+as `UV_PYTHON`. Only a single Python version is supported; multiple fallback versions and the asdf
+`ref:`, `path:`, and `system` forms are ignored with a warning.
 
 ```yaml
 - name: Install uv based on the version defined in pyproject.toml

docs/environment-and-tools.md

@@ -131,7 +131,7 @@ If you want to ignore this, set the `ignore-empty-workdir` input to `true`.
 
 This action sets several environment variables that influence uv's behavior and can be used by subsequent steps:
 
-- `UV_PYTHON`: Set when `python-version` input is specified. Controls which Python version uv uses.
+- `UV_PYTHON`: Set when `python-version` is specified or the selected `.tool-versions` file contains a supported `python` entry. Controls which Python version uv uses.
 - `UV_CACHE_DIR`: Set when caching is enabled (unless already configured in uv config files). Controls where uv stores its cache.
 - `UV_TOOL_DIR`: Set when `tool-dir` input is specified. Controls where uv installs tool environments.
 - `UV_TOOL_BIN_DIR`: Set when `tool-bin-dir` input is specified. Controls where uv installs tool binaries.
@@ -142,6 +142,7 @@ This action sets several environment variables that influence uv's behavior and
 
 - `UV_NO_MODIFY_PATH`: If set, prevents the action from modifying PATH. Cannot be used with `activate-environment`.
 - `UV_CACHE_DIR`: If already set, the action will respect it instead of setting its own cache directory.
+- `UV_PYTHON`: If already set and `python-version` is not specified, the action will respect it instead of using the `python` entry from `.tool-versions`.
 
 ```yaml
 - name: Example using environment variables