The detached watcher runs as `<python> -c <program>`, so `hermes_cli` resolved only
because update_completion happened to spawn it with cwd=<checkout>. The program now puts
the checkout on sys.path itself, and the bare-Python test runs it from an unrelated cwd
(red without the sys.path line).
Also drops the two unused re-export aliases in gateway.status (`_posix_is_zombie`,
`_pid_exists_win32_ctypes`): nothing imports them and neither is in the old-updater
compat surface.
After the package-manager handoff, hermes update finishes on the bare store
interpreter and spawns the detached restart watcher as sys.executable -c.
The watcher imported gateway.status (utils -> hermes_yaml -> ruamel) and
hermes_cli.config, died with ModuleNotFoundError before relaunching, and
left every manually started gateway down after the update (gated on #124649).
Move the stdlib liveness probe (zombie-aware POSIX kill(0), Windows
OpenProcess) into hermes_cli._subprocess_compat, have gateway.status's
fallback delegate to it, and import only stdlib-backed modules in the
watcher.
Fixes#124649.
