windows: replace local pipe spawning

[anp-oai]

Intent

The Windows non-TTY Tokio launch path cannot guarantee Job Object assignment before a child executes. Unified exec needs a raw launcher that preserves pipe behavior while enforcing suspended creation, assignment, and resume.

Implementation

• Add RAII stdin, stdout, and stderr pipes with inheritance cleared on parent-side handles.
• Pass only child-side handles through PROC_THREAD_ATTRIBUTE_HANDLE_LIST.
• Create the job before CreateProcessW, launch with CREATE_SUSPENDED, then assign and resume through SuspendedProcess.
• Adapt blocking pipe readers and writers into the existing SpawnedProcess channels while preserving split stdout/stderr and tail draining.
• Close the job before publishing root exit and terminate it for session termination or drop.
• Add root-exit, explicit-termination, and session-drop descendant tests.

Manual validation

• Ran just test -p codex-utils-pty on Linux.
• Cross-compiled the codex-utils-pty Windows test target with cargo +1.95.0 check --target x86_64-pc-windows-msvc -p codex-utils-pty --tests.

Stack

1. job primitives → 2. local ConPTY → 3. restricted sessions → 4. elevated runner → 5. command preparation → 6. raw pipe launcher → 7. command parity