Feature request: get

The yin to `set`'s yang:

```
renv -e production get service.env.USE_CACHE
```

This would be an alias for `config ---key`:

```
renv -e production config --key service.env.USE_CACHE
==> production.service.env.USE_CACHE
0 't'
1 'r'
2 'u'
3 'e'
```

Start by tracing the renv CLI entry point for `config --key` and how it handles the `service.env.USE_CACHE` key. Add the `get` command as an alias with the same environment and key behavior, then verify that `renv -e production get service.env.USE_CACHE` produces the existing config output.

Written by the indexing model from the issue text.