fix(security): require confirm and OS trash for delete_asset (#140)

[Daliys]

Summary

Implements security controls and path safety for delete_asset as requested in #140:

• Requires explicit confirm: true parameter for delete_asset calls.
• Moves deleted assets to OS Trash (AssetDatabase.MoveAssetToTrash) with AssetDatabase.DeleteAsset fallback.
• Strictly validates asset paths: restricts deletion to subpaths under Assets/, blocking root folder deletion (Assets, Packages, ProjectSettings) and system folders (ProjectSettings/, Packages/).
• Protects destination paths in move_asset and copy_asset from modifying system folders (ProjectSettings/, Packages/).
• Prevents direct deletion of .meta files and handles non-existent asset paths gracefully.
• Adds comprehensive unit tests covering all path edge cases.

Closes #140