## Why
Default read-only protections for project metadata should apply when paths such
as `.git`, `.agents`, and `.codex` exist, without causing sandbox setup to
materialize missing paths as ACL targets.
## What changed
- Add an optional `missing_path_behavior` to filesystem sandbox entries and
mark default project-metadata protections with `skip`.
- Preserve the behavior through permission transforms and exec/MCP protocol
serialization while keeping existing path wire variants stable.
- Ignore skip-missing entries when projecting configuration or Windows sandbox
overrides, while retaining explicit metadata carveouts.
## Testing
- Cover protocol round trips for path and special-path entries.
- Verify default metadata protections and Windows explicit carveout handling.
GitOrigin-RevId: 6df13dadacdd131c44aab9f15a967c81051355c1