fix: resolve 3 critical review findings

C1: mnt_extract.go rename bug — source path was mutated before
    reading from fs.FS. Now uses separate sourcePath/targetPath.

C2: cli_command.go os.Stderr = nil — replaced with
    flags.SetOutput(io.Discard). No more global nil stderr.

C3: Cli() returned nil — now initialised in New() with
    NewCliApp("", "", "").

Found by opus code-reviewer agent (final review pass).

Co-Authored-By: Virgil <virgil@lethean.io>

pkg/core/cli_command.go

@@ -3,6 +3,7 @@ package core
 import (
 	"errors"
 	"flag"
+	"io"
 	"fmt"
 	"os"
 	"reflect"
@@ -75,11 +76,9 @@ func (c *Command) setApp(app *CliApp) {
 // parseFlags parses the given flags
 func (c *Command) parseFlags(args []string) error {
 	// Parse flags
-	tmp := os.Stderr
-	os.Stderr = nil
-	defer func() {
-		os.Stderr = tmp
-	}()
+	// Suppress flag parse errors to stderr
+	
+	c.flags.SetOutput(io.Discard)
 
 	// Credit: https://stackoverflow.com/a/74146375
 	var positionalArgs []string

pkg/core/core.go

@@ -33,6 +33,7 @@ func New(opts ...Option) (*Core, error) {
 		io:    defaultIO,
 		etc:   NewEtc(),
 		crash: NewCrashHandler(),
+		cli:   NewCliApp("", "", ""),
 		svc:   newServiceManager(),
 	}
 	c.bus = newMessageBus(c)

pkg/core/mnt_extract.go

@@ -135,11 +135,12 @@ func Extract(fsys fs.FS, targetDir string, data any, opts ...ExtractOptions) err
 
 	// Copy standard files
 	for _, path := range standardFiles {
+		targetPath := path
 		name := filepath.Base(path)
 		if renamed := opt.RenameFiles[name]; renamed != "" {
-			path = filepath.Join(filepath.Dir(path), renamed)
+			targetPath = filepath.Join(filepath.Dir(path), renamed)
 		}
-		target := renderPath(filepath.Join(targetDir, path), data)
+		target := renderPath(filepath.Join(targetDir, targetPath), data)
 		if err := copyFile(fsys, path, target); err != nil {
 			return err
 		}