fix: Address various issues in compile, run, and matrix tests

This commit addresses three separate issues:

1.  **cmd/compile.go**: The `Borgfile` parsing now returns an error for
    unknown instructions instead of silently ignoring them.
2.  **cmd/run_test.go**: The `execCommand` mock is now properly cleaned
    up after the test, preventing it from leaking into other tests.
3.  **pkg/matrix/matrix_test.go**: The EOF check in the tar reader loop
    now uses a direct error comparison (`io.EOF`) instead of a string
    comparison.

cmd/compile.go

@@ -44,6 +44,8 @@ var compileCmd = &cobra.Command{
 					return err
 				}
 				m.RootFS.AddData(dest, data)
+			default:
+				return fmt.Errorf("unknown instruction: %s", parts[0])
 			}
 		}
 

cmd/run_test.go

@@ -65,7 +65,11 @@ func TestRunCmd_Good(t *testing.T) {
 	}
 
 	// Mock the exec.Command function.
+	origExecCommand := execCommand
 	execCommand = helperProcess
+	t.Cleanup(func() {
+		execCommand = origExecCommand
+	})
 
 	// Run the run command.
 	rootCmd := NewRootCmd()

pkg/matrix/matrix_test.go

@@ -3,6 +3,7 @@ package matrix
 import (
 	"archive/tar"
 	"bytes"
+	"io"
 	"testing"
 
 	"github.com/Snider/Borg/pkg/datanode"
@@ -60,7 +61,7 @@ func TestToTar(t *testing.T) {
 	for {
 		header, err := tr.Next()
 		if err != nil {
-			if err.Error() == "EOF" {
+			if err == io.EOF {
 				break
 			}
 			t.Fatalf("failed to read tar header: %v", err)