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

[fengmk2]

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:

# .yarnrc.yml
nodeLinker: pnp

Run migrate with:

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.