Test: add unit tests for restoreUserDir

[cicalese]

Summary

Add unit tests for the restoreUserDir function in internal/devmode/devmode.go.

What to test

This function reverses consolidateUserDir — it removes the symlink and restores the user directory as a real directory with its original contents.

Test cases:

• Symlink exists pointing to code dir — should remove symlink and restore directory
• Real directory exists (not a symlink) — should handle gracefully
• Directory does not exist — should be a no-op
• Source directory in code dir has contents — should copy them back

How to test

Use t.TempDir() to create temporary directory structures. No Docker or orchestrator needed.

Context

Part of the test coverage initiative (#241).

[cicalese]

Done in #732.