Isolate Git test fixtures from the developer's global git config #425

TestWorktreeStorageMissingMainPreservesCanonicalIdentity fails on any machine whose global git config sets worktree.useRelativePaths = true. Found on 22 October while rebasing #422: the test fails on main (02f36e9, and also 182157e) on such a machine and passes with GIT_CONFIG_GLOBAL=/dev/null. Run alone or with the other worktree storage tests, it fails every time, so a developer checking worktree behavior gets a false failure. Two full make verify-local runs on the same machine passed it, for a reason not yet understood.

Steps to reproduce

1. Set git config --global worktree.useRelativePaths true.
2. Run env -u LOAF_DB go test ./internal/cli -run TestWorktreeStorageMissingMainPreservesCanonicalIdentity -count=1.
3. The relative-pointer-true case fails because filepath.Rel cannot relate the already-relative gitdir path to the absolute worktree path. With GIT_CONFIG_GLOBAL=/dev/null the test passes.

Definition of Done

• The test helpers that create Git fixtures run git without reading the developer's global or system config, so the suite gives the same result on every machine.
• TestWorktreeStorageMissingMainPreservesCanonicalIdentity passes with worktree.useRelativePaths = true set globally and with it unset.
• Product behavior is unchanged.

Out of Scope

• Changing how Loaf resolves relative gitdir pointers; the test's relative case already covers that once its setup works.

Verification

• The test passes with worktree.useRelativePaths = true in the global config and with it unset.
• go test ./internal/cli -run TestWorktreeStorage -count=1 and make verify-local both pass on a machine with that setting.