A root-launched CLI session can leak /root into the terminal cwd state a
non-root gateway/cron process later resolves (#65583). os.path.isdir('/root')
is True for a non-root user — stat only needs search permission on / — so
_resolve_safe_cwd returned it and subprocess.Popen(cwd='/root') died with
PermissionError: [Errno 13], failing EVERY cron job's terminal/file/search
tool on every command until restart.
_resolve_safe_cwd now requires X_OK (new _cwd_usable helper) and climbs to
the nearest enterable ancestor, logging a WARNING that names the leak class
when an existing-but-denied cwd is skipped. Missing-cwd recovery (#17558)
behavior unchanged.
E2E-verified: LocalEnvironment constructed with an unenterable cwd now runs
commands from the fallback directory instead of raising.
