test(import): fully isolate git config in author fallback test

TestRun_UnconfiguredGitIdentityFallsBackToDefaults overrode only HOME and
XDG_CONFIG_HOME, but GetGitAuthorFromRepo resolves GlobalScope through
go-git's Auto loader, which also honors GIT_CONFIG_GLOBAL and system
/etc/gitconfig. On any machine/CI where one of those carries a git identity
the "Unknown"/"unknown@local" assertion would flake.

Neutralize every global source the way the checkpoint package's pointHomeAt
helper already does: set GIT_CONFIG_NOSYSTEM=1 and unset GIT_CONFIG_GLOBAL.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Entire-Checkpoint: 01KY9MM18RHEBYFH4Y4QVJK7RT

Author: gtrrz-victor

cmd/entire/cli/agentimport/agentimport_test.go

@@ -438,11 +438,22 @@ func TestRun_StampsImporterGitAuthorOnCheckpointCommit(t *testing.T) {
 // same "Unknown"/"unknown@local" default checkpoint.GetGitAuthorFromRepo
 // already applies elsewhere, rather than an empty one.
 func TestRun_UnconfiguredGitIdentityFallsBackToDefaults(t *testing.T) {
-	// Cannot use t.Parallel(): isolates HOME via t.Setenv so this repo's
-	// global git config resolution can't see the developer's real ~/.gitconfig.
+	// Cannot use t.Parallel(): isolates git config resolution via t.Setenv so
+	// this repo can't see any real identity. GetGitAuthorFromRepo resolves
+	// GlobalScope through go-git's Auto loader, which reads all of git's global
+	// sources; neutralize every one or the fallback assertion is flaky wherever
+	// an identity is configured (~/.gitconfig, XDG, GIT_CONFIG_GLOBAL, or system
+	// /etc/gitconfig). Mirrors the checkpoint package's pointHomeAt helper.
 	home := t.TempDir()
 	t.Setenv("HOME", home)
 	t.Setenv("XDG_CONFIG_HOME", "")
+	t.Setenv("GIT_CONFIG_NOSYSTEM", "1")
+	// t.Setenv registers restoration of the original value; unset it for the
+	// test since an empty GIT_CONFIG_GLOBAL disables global config entirely.
+	t.Setenv("GIT_CONFIG_GLOBAL", "")
+	if err := os.Unsetenv("GIT_CONFIG_GLOBAL"); err != nil {
+		t.Fatal(err)
+	}
 
 	repoDir := t.TempDir()
 	repo, err := git.PlainInit(repoDir, false)