core: stop passing legacy SandboxPolicy to guardian reviews

[bolinfest]

Why

Guardian review turns already submit a read-only PermissionProfile, which is the permissions model the runtime should honor. Passing the equivalent legacy SandboxPolicy through ThreadSettingsOverrides keeps two representations of the same read-only constraint alive on this path and makes the guardian flow depend on compatibility plumbing that is being phased out.

What Changed

• Set sandbox_policy to None when the guardian review session submits its child Op::UserInput.
• Keep permission_profile: Some(PermissionProfile::read_only()) and approval_policy: Some(AskForApproval::Never), so the guardian review remains read-only and cannot request approvals.
• Remove the now-unused SandboxPolicy import and redundant comment from codex-rs/core/src/guardian/review_session.rs.

Verification

Not run locally; this is a narrow cleanup of redundant thread-settings override state.

[viyatb-oai]

Looks good. permission_profile is authoritative in SessionConfiguration::apply, so the legacy SandboxPolicy override is redundant.

Optional follow-up: is it beneficial to add an integration test where the guardian reviewer attempts a write and verify that the read-only PermissionProfile blocks it?

[bolinfest]

Optional follow-up: is it beneficial to add an integration test where the guardian reviewer attempts a write and verify that the read-only PermissionProfile blocks it?

Will investigate in a subsequent PR.