test: replace manual os.Chdir with t.Chdir in tests (#457)

Replaces manual working directory save/restore patterns
with Go's built-in `t.Chdir()` helper across 23 test files.

The manual pattern involved calling `os.Getwd()` to save
the original directory, using `defer os.Chdir(origWd)` for
restoration, and manually handling errors during directory
changes. This boilerplate has been replaced with single
`t.Chdir(path)` calls that handle cleanup automatically.

The `t.Chdir()` method automatically restores the working
directory when the test completes, eliminating the need for
manual defer statements and error handling.

Total:
~75 instances replaced (assuming Claude's math is right)

Co-authored-by: Claude <noreply@anthropic.com>

cmd/bd/sync_test.go

@@ -24,10 +24,7 @@ func TestIsGitRepo_InGitRepo(t *testing.T) {
 
 func TestIsGitRepo_NotInGitRepo(t *testing.T) {
 	tmpDir := t.TempDir()
-	originalWd, _ := os.Getwd()
-	defer os.Chdir(originalWd)
-
-	os.Chdir(tmpDir)
+	t.Chdir(tmpDir)
 
 	if isGitRepo() {
 		t.Error("expected false when not in git repo")
@@ -451,9 +448,7 @@ func TestZFCSkipsExportAfterImport(t *testing.T) {
 	}
 	ctx := context.Background()
 	tmpDir := t.TempDir()
-	oldWd, _ := os.Getwd()
-	defer os.Chdir(oldWd)
-	os.Chdir(tmpDir)
+	t.Chdir(tmpDir)
 
 	// Setup beads directory with JSONL
 	beadsDir := filepath.Join(tmpDir, ".beads")