feat(interpreter): exec with command argument — execute and don't return

[chaliy]

Summary\n\n- exec cmd args... now resolves and executes the command, then exits the current script/shell with its exit code\n- Subsequent statements after exec are not executed (uses ControlFlow::Return)\n- FD-only exec (no command) continues to work as before\n\nCloses #794\nCloses #797\n\n## Test plan\n\n- [x] exec replaces execution (subsequent statements skipped)\n- [x] exec propagates exit code\n- [x] exec with builtin command\n- [x] exec passes arguments\n- [x] exec FD redirections still work\n- [x] cargo test --all-features green\n- [x] cargo clippy and cargo fmt clean