Closes#1270's regex-only test gap. PR #1270 asserted that codex/SKILL.md's
`codex exec resume` invocation drops -C/-s and uses sandbox_mode config.
That regex catches the skill template regressing, but not codex CLI itself
flipping flag semantics again.
This test probes `codex exec resume --help` and asserts the surface gstack
relies on: -c/sandbox_mode is accepted, top-level -C is absent. Skips
silently when codex isn't on PATH, so dev machines without codex installed
never see it fail.
Garry Tan