test: gbrain-missing PATH case leaks host /opt/homebrew/bin #2255

Reproduction\n\nEnvironment: macOS arm64 with gbrain installed in /opt/homebrew/bin, origin/main 83238f9ba28b9a5dfa3405fa625b70f7b5b99d39.\n\nRun:\n\n bun test test/gstack-gbrain-source-wireup.test.ts\n\nThe test named "--strict + gbrain missing on PATH: exits 2" fails with expected 2, received 1.\n\nThe fixture says its fake bin is empty but constructs PATH with /opt/homebrew/bin. On a developer host where gbrain is installed there, the supposedly missing executable is visible and the test is not hermetic.\n\n### Expected\n\nThe missing-PATH fixture should exclude host package-manager directories or otherwise shadow gbrain deterministically.