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/version_test.go

@@ -266,7 +266,10 @@ func TestGetSecretWithVersion(t *testing.T) {
 
 func TestVersionCommandStructure(t *testing.T) {
 	// Test that version commands are properly structured
-	cli := NewCLIInstance()
+	cli, err := NewCLIInstance()
+	if err != nil {
+		t.Fatalf("failed to initialize CLI: %v", err)
+	}
 	cmd := VersionCommands(cli)
 
 	assert.Equal(t, "version", cmd.Use)