Two thinking tests wrote fake CLI scripts via os.WriteFile and immediately
execed them. Under t.Parallel() with the rest of pkg/agent, a sibling
test's concurrent fork can inherit our still-open write fd, so Linux
returns ETXTBSY at exec time (Go #22315). CI hit this on main as
"TestRunCodexDebugModels_ArgvSeenByBinary: fork/exec ...: text file busy".
Switch both call sites to the existing writeTestExecutable helper, which
holds syscall.ForkLock across OpenFile→Write→Close so no concurrent fork
can inherit the write fd. Same pattern the rest of the package already
uses (kimi, kiro, codex, claude tests).
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