## Why
Unix command hooks use a `pre_exec` callback to detach from the controlling terminal, forcing the launcher to fork. Native spawn attributes can create the detached session without that callback.
## What changed
- Route Unix hooks through `codex_utils_pty::Command` with `ProcessMode::NewSession`.
- Extend the native `posix_spawn` launcher to detached Linux sessions, retaining compatibility fallbacks for unsupported libc features and executable handling.
- Preserve the session environment snapshot, hook overrides, and restricted-variable filtering.
## Testing
Add regression coverage for launches without parent forks, terminal detachment, environment and argument handling, Linux `PATH` lookup, and executable compatibility. Cover process-group cleanup on cancellation and timeout, including descendants holding output pipes open, and preservation of background descendants after completed hooks.
GitOrigin-RevId: dbe2cf12d84aa79868cfb55ace40c7d5b5282703
## What changed
Add a crate-private `ChildDropPolicy` to choose whether dropping a child kills it or only reaps it after exit. Keep `KillAndReap` as the default.
For `ReapOnly` on Unix, transfer dropped children to a shared reaper thread that operates independently of the Tokio runtime. Support both Tokio children and native macOS children, without blocking drop or letting a live child delay reaping others.
## Testing
Add an isolated regression test verifying that reap-only children survive handle drop after runtime shutdown, are reaped after exit, and do not prevent other exited children from being reaped.
GitOrigin-RevId: 5ec639597dba7f3b61694d6d465991e054ae3397
Charlie Marsh