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/secrets_size_test.go

@@ -113,7 +113,10 @@ func TestAddSecretVariousSizes(t *testing.T) {
 			cmd.SetIn(stdin)
 
 			// Create CLI instance
-			cli := NewCLIInstance()
+			cli, err := NewCLIInstance()
+			if err != nil {
+				t.Fatalf("failed to initialize CLI: %v", err)
+			}
 			cli.fs = fs
 			cli.stateDir = stateDir
 			cli.cmd = cmd
@@ -230,7 +233,10 @@ func TestImportSecretVariousSizes(t *testing.T) {
 			cmd := &cobra.Command{}
 
 			// Create CLI instance
-			cli := NewCLIInstance()
+			cli, err := NewCLIInstance()
+			if err != nil {
+				t.Fatalf("failed to initialize CLI: %v", err)
+			}
 			cli.fs = fs
 			cli.stateDir = stateDir
 
@@ -318,7 +324,10 @@ func TestAddSecretBufferGrowth(t *testing.T) {
 			cmd.SetIn(stdin)
 
 			// Create CLI instance
-			cli := NewCLIInstance()
+			cli, err := NewCLIInstance()
+			if err != nil {
+				t.Fatalf("failed to initialize CLI: %v", err)
+			}
 			cli.fs = fs
 			cli.stateDir = stateDir
 			cli.cmd = cmd
@@ -377,7 +386,10 @@ func TestAddSecretStreamingBehavior(t *testing.T) {
 	cmd.SetIn(slowReader)
 
 	// Create CLI instance
-	cli := NewCLIInstance()
+	cli, err := NewCLIInstance()
+	if err != nil {
+		t.Fatalf("failed to initialize CLI: %v", err)
+	}
 	cli.fs = fs
 	cli.stateDir = stateDir
 	cli.cmd = cmd