scaffold a hermetic git environment for tests that shell out to git: any test invoking git as a subprocess can read the developer's real ~/.gitconfig (identity, aliases, includeIf, hooks path) — non-determinism and identity leakage into fixtures — or, via one un-scoped call (missing -C/cwd) or corrupted repo state, mutate the ambient repo's config, refs, or history. Per-call temp-repo scoping is defence-in-breadth only; the robust fix also isolates the git environment. Ship a shared hermetic-git helper in the generated test scaffolding, per target language (Go gitIsolatedEnv(), pytest fixture, shell shim), that pins HOME and XDG_CONFIG_HOME to a per-test temp dir, sets GIT_CONFIG_GLOBAL/GIT_CONFIG_SYSTEM to /dev/null (or GIT_CONFIG_NOSYSTEM=1), supplies identity via GIT_AUTHOR_*/GIT_COMMITTER_* env rather than git config, sets GIT_TERMINAL_PROMPT=0 and core.hooksPath=/dev/null, and is always combined with scoping to the temp repo — documented in the scaffolded repo's testing conventions/AGENTS.md so it is the path of least resistance. Optional enforcement: a lint/pre-commit check flagging git subprocesses in test code that bypass the helper, turning convention into guarantee. Exactly the cross-repo hygiene the scaffolder exists to standardise; removes a whole class of flaky-test and repo-pollution bugs.

scaffold a hermetic git environment for tests that shell out to git: any test invoking git as a subprocess can read the developer's real ~/.gitconfig (identity, aliases, includeIf, hooks path) — non-determinism and identity leakage into fixtures — or, via one un-scoped call (missing -C/cwd) or corrupted repo state, mutate the ambient repo's config, refs, or history. Per-call temp-repo scoping is defence-in-breadth only; the robust fix also isolates the git environment. Ship a shared hermetic-git helper in the generated test scaffolding, per target language (Go gitIsolatedEnv(), pytest fixture, shell shim), that pins HOME and XDG_CONFIG_HOME to a per-test temp dir, sets GIT_CONFIG_GLOBAL/GIT_CONFIG_SYSTEM to /dev/null (or GIT_CONFIG_NOSYSTEM=1), supplies identity via GIT_AUTHOR_/GIT_COMMITTER_ env rather than git config, sets GIT_TERMINAL_PROMPT=0 and core.hooksPath=/dev/null, and is always combined with scoping to the temp repo — documented in the scaffolded repo's testing conventions/AGENTS.md so it is the path of least resistance. Optional enforcement: a lint/pre-commit check flagging git subprocesses in test code that bypass the helper, turning convention into guarantee. Exactly the cross-repo hygiene the scaffolder exists to standardise; removes a whole class of flaky-test and repo-pollution bugs.

Reproduction mechanism confirmed (relayed 2026-07-10, external agent): any hook-context run leaks GIT_DIR into test subprocesses, redirecting their git init/commit onto the real repo. The hook-side unset GIT_DIR protects this repo's gate only; the tests themselves must stop trusting inherited git env — the hermetic helper above must clear GIT_DIR (and GIT_WORK_TREE/GIT_INDEX_FILE) in addition to the config/identity isolation already listed. The global ~/.githooks dispatcher has the same exposure for every other repo whose gate runs git-spawning tests, which raises the cross-repo urgency of shipping the shared helper.