fix: Preserve source dependencies when adding workspace deps in turbo-gen

[sleitor]

Closes #11505

Summary

Fix turbo gen workspace --copy to preserve the source package's existing dependencies when the user chooses to add workspace dependencies.

Problem

When copying a workspace and selecting "Add workspace dependencies", the copied package's original dependencies, devDependencies, etc. are completely replaced by only the selected workspace packages. All original deps from the source are lost.

Root Cause

In packages/turbo-gen/src/generators/copy.ts, the dependency update logic uses direct assignment:

packageJson[group] = deps;

This overwrites the entire dependency group instead of merging.

Fix

Merge the workspace dependencies with the existing ones:

packageJson[group] = { ...packageJson[group], ...deps };

Workspace deps take precedence (override) if there's a conflict, which is the expected behavior when explicitly adding them.

[vercel]

Someone is attempting to deploy a commit to the Vercel Team on Vercel.

A member of the Team first needs to authorize it.

[sleitor]

cc @anthonyshew — would appreciate a review when you get a chance!

[anthonyshew]

Thanks!