🧹 [code health improvement] simplify directory traversal checks in renv-cache

[MiguelRodo]

🎯 What: Simplified the directory path traversal validation checks in src/renv-cache/install.sh's empty_dir and rm_dirs functions.
💡 Why: The previous implementation used six explicit string match conditions connected by logical ORs, making the code highly verbose. The refactor replaces this with a single, concise extended regular expression.
✅ Verification: Verified that the regex preserves the exact original behavior via standalone bash testing scripts: it correctly blocks path traversal values (e.g., /foo/../bar, /.) while safely allowing valid hidden folders (e.g., /.hidden). Passed syntax checks with bash -n.
✨ Result: A significant improvement in maintainability and readability by cutting down duplicated, verbose conditional logic.

---

PR created automatically by Jules for task 14194312082766388640 started by @MiguelRodo

[google-labs-jules]

👋 Jules, reporting for duty! I'm here to lend a hand with this pull request.

When you start a review, I'll add a 👀 emoji to each comment to let you know I've read it. I'll focus on feedback directed at me and will do my best to stay out of conversations between you and other bots or reviewers to keep the noise down.

I'll push a commit with your requested changes shortly after. Please note there might be a delay between these steps, but rest assured I'm on the job!

For more direct control, you can switch me to Reactive Mode. When this mode is on, I will only act on comments where you specifically mention me with @jules. You can find this option in the Pull Request section of your global Jules UI settings. You can always switch back!

New to Jules? Learn more at jules.google/docs.

---

For security, I will only act on instructions from the user who triggered this task.