fix(migrate): detect Yarn PnP config even when env overrides nodeLinker

`vp migrate` currently treats `YARN_NODE_LINKER=node-modules` as enough to skip Yarn PnP handling. However, if the repository itself has `.yarnrc.yml` with `nodeLinker: pnp`, migration can complete successfully while leaving unsupported PnP config in the project. Future installs without that environment override will return to PnP mode.

Expected behavior:

- If `.yarnrc.yml` explicitly sets `nodeLinker: pnp`, migration should not silently pass just because `YARN_NODE_LINKER=node-modules` is set.
- Preferably, migration should stop and ask the user to change the project config manually, or apply the existing node-modules conversion flow if that is the intended behavior.
- `YARN_NODE_LINKER=pnp` should still hard-block migration as today.

Suggested test:

Create a Yarn Berry fixture with:

```yaml
# .yarnrc.yml
nodeLinker: pnp
```

Run migrate with:

```bash
YARN_NODE_LINKER=node-modules vp migrate --no-interactive
```

Assert that migration does not report the project as successfully migrated while leaving `nodeLinker: pnp` unchanged.