The Mac and Linux implementations of the sandbox recently added write
protections for `.codex` and `.agents` subdirectories in all writable
roots. When adding documentation for this, I noticed that this change
was never made for the Windows sandbox.
Summary
- make compute_allow_paths treat .codex/.agents as protected alongside
.git, and cover their behavior in new tests
- wire protect_workspace_agents_dir through the sandbox lib and setup
path to apply deny ACEs when `.agents` exists
- factor shared ACL logic for workspace subdirectories