Support symlink path placeholders

[somework]

Summary

• allow symlink targets and links to expand %project-dir%, %vendor-dir%, and %env(NAME)% placeholders before validation
• permit absolute paths produced via placeholders while keeping default relative-only validation otherwise
• cover new placeholder behaviour with unit tests and document the syntax in the README
• add regression tests for empty environment placeholder expansion and relative vendor-dir configuration

Testing

• vendor/bin/phpunit

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https://chatgpt.com/codex/tasks/task_e_68d58a0d1c788320852f048e0853f818