Git defaults the initial branch to 'master' unless init.defaultBranch is set —
true on Linux CI runners but typically overridden to 'main' on developer macOS
machines. That host gap made git-worktree tests assuming 'main' (the
shared-branch-group reliability suite in engine shard 2/2) pass locally but
fail only in CI with 'fatal: path ... does not exist in main'.
Set init.defaultBranch=main for every test process via GIT_CONFIG_* env vars in
the shared core test setup (inherited by all child git invocations, without
mutating the developer's global config). Appends rather than clobbers any
pre-existing GIT_CONFIG_COUNT.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
3432e0615c