Move macOS sandbox builders into codex-sandboxing (#15593)

## 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

codex-rs/core/src/tools/runtimes/shell/unix_escalation_tests.rs

@@ -20,8 +20,6 @@ use crate::protocol::GranularApprovalConfig;
 use crate::protocol::ReadOnlyAccess;
 use crate::protocol::SandboxPolicy;
 use crate::sandboxing::SandboxPermissions;
-#[cfg(target_os = "macos")]
-use crate::seatbelt::MACOS_PATH_TO_SEATBELT_EXECUTABLE;
 use crate::skills::SkillMetadata;
 use codex_execpolicy::Decision;
 use codex_execpolicy::Evaluation;
@@ -40,6 +38,8 @@ use codex_protocol::permissions::FileSystemSandboxPolicy;
 use codex_protocol::permissions::FileSystemSpecialPath;
 use codex_protocol::permissions::NetworkSandboxPolicy;
 use codex_protocol::protocol::SkillScope;
+#[cfg(target_os = "macos")]
+use codex_sandboxing::seatbelt::MACOS_PATH_TO_SEATBELT_EXECUTABLE;
 use codex_shell_escalation::EscalationExecution;
 use codex_shell_escalation::EscalationPermissions;
 use codex_shell_escalation::ExecResult;