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/init.go

@@ -1,6 +1,7 @@
 package cli
 
 import (
+	"log"
 	"fmt"
 	"log/slog"
 	"os"
@@ -27,7 +28,10 @@ func NewInitCmd() *cobra.Command {
 
 // RunInit is the exported function that handles the init command
 func RunInit(cmd *cobra.Command, _ []string) error {
-	cli := NewCLIInstance()
+	cli, err := NewCLIInstance()
+	if err != nil {
+		log.Fatalf("failed to initialize CLI: %v", err)
+	}
 
 	return cli.Init(cmd)
 }