feat: use shell policy in shell snapshot (#11759)

Honor `shell_environment_policy.set` even after a shell snapshot

codex-rs/core/src/unified_exec/process_manager.rs

@@ -622,6 +622,7 @@ impl UnifiedExecProcessManager {
             command: request.command.clone(),
             cwd,
             env,
+            explicit_env_overrides: context.turn.shell_environment_policy.r#set.clone(),
             network: request.network.clone(),
             tty: request.tty,
             sandbox_permissions: request.sandbox_permissions,