## Summary
- move macOS permission merging/intersection logic and tests from
`codex-core` into `codex-sandboxing`
- move seatbelt policy builders, permissions logic, SBPL assets, and
their tests into `codex-sandboxing`
- keep `codex-core` owning only the seatbelt spawn wrapper and switch
call sites to import the moved APIs directly
## Notes
- no re-exports added
- moved the seatbelt tests with the implementation so internal helpers
could stay private
- local verification is still finishing while this PR is open
## Summary
- add a new `codex-sandboxing` crate for sandboxing extraction work
- move the pure Linux sandbox argv builders and their unit tests out of
`codex-core`
- keep `core::landlock` as the spawn wrapper and update direct callers
to use `codex_sandboxing::landlock`
## Testing
- `cargo test -p codex-sandboxing`
- `cargo test -p codex-core landlock`
- `cargo test -p codex-cli debug_sandbox`
- `just argument-comment-lint`
## Notes
- this is step 1 of the move plan aimed at minimizing per-PR diffs
- no re-exports or no-op proxy methods were added
