os/exec: commands cannot start with standard file descriptors on Linux and Darwin

My CLI needs to run child commands and read their output. exec.Command("/bin/echo", "ok").Output() cannot do this with the current process implementation.

In dev at 93940cb6, Linux rejects ProcAttr.Files with files setting not implemented. os/exec supplies these files for standard input and output. Darwin returns not implemented. Process wait and exit status methods are also stubs. The command should return ok\n and a successful exit status.

This is a source check from 18 September 2026. #632 closed on 27 November 2024 and #4471 closed on 4 October 2025 after earlier process work. The remaining file descriptor and process state gaps need this follow-up.

#5634 contains the proposed implementation and tests. It closed on 17 September with a request for an issue. Darwin also needs #5612 and #5694 (PR #5636). Concurrent pipe and process creation needs #5692 (PR #5630). Interruptible pipe I/O and WaitDelay remain separate work.

Replacement draft PR #5699.