fix(init): auto-fix git symlinks when core.symlinks is false

[T4ko0522]

Problem

This repository contains git symlinks in packages/core/ that reference files in the upstream vite/
directory:

• packages/core/vite-rolldown.config.ts → ../../vite/packages/vite/rolldown.config.ts
• packages/core/rollupLicensePlugin.ts → ../../vite/packages/vite/rollupLicensePlugin.ts

In a fresh Windows clone of this repository, core.symlinks can be false. In that case, git checks
out these entries as plain text files containing the target path instead of actual symlinks. This
causes the build to fail with Unexpected token, because the files are imported as TypeScript modules
but only contain a path string.

Solution

Add a _fix_symlinks recipe to justfile and run it as part of just init. The recipe checks
core.symlinks, and when it is not true, it enables it and re-checks out all tracked symlink
entries.

Both unix and Windows implementations are included.

The recipe is idempotent: when core.symlinks=true, it does nothing.

Test plan

• Verified on Windows with a fresh git clone of this repository that core.symlinks=false and the
  symlinks are checked out incorrectly
• Ran just _fix_symlinks on Windows via PowerShell and confirmed the symlinks were restored
• Ran the unix _fix_symlinks commands manually in Git Bash and confirmed the symlinks were restored
• Confirmed idempotency: running the recipe again with core.symlinks=true is a no-op

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[fengmk2]

@cursor review

[fengmk2]

@T4ko0522 Thanks