refactor: return errors from NewCLIInstance instead of panicking

Change NewCLIInstance() and NewCLIInstanceWithFs() to return
(*Instance, error) instead of panicking on DetermineStateDir failure.

Callers in RunE contexts propagate the error. Callers in command
construction (for shell completion) use log.Fatalf. Test callers
use t.Fatalf.

Addresses review feedback on PR #18.

internal/cli/cli_test.go

@@ -25,7 +25,10 @@ func TestCLIInstanceStateDir(t *testing.T) {
 func TestCLIInstanceWithFs(t *testing.T) {
 	// Test creating CLI instance with custom filesystem
 	fs := afero.NewMemMapFs()
-	cli := NewCLIInstanceWithFs(fs)
+	cli, err := NewCLIInstanceWithFs(fs)
+	if err != nil {
+		t.Fatalf("failed to initialize CLI: %v", err)
+	}
 
 	// The state directory should be determined automatically
 	stateDir := cli.GetStateDir()