Fix embed.FS path separator on Windows

Use path.Join (always '/') for embed.FS reads, filepath.Join only for OS paths.
filepath.Join on Windows produces backslashes which embed.FS doesn't accept,
causing "failed to extract scripts" on startup.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/runner/runner.go

@@ -8,6 +8,7 @@ import (
 	"fmt"
 	"os"
 	"os/exec"
+	"path"
 	"path/filepath"
 	"strings"
 	"time"
@@ -277,7 +278,8 @@ func ExtractScripts(fs interface{ ReadDir(string) ([]os.DirEntry, error); ReadFi
 		if e.IsDir() {
 			continue
 		}
-		data, err := fs.ReadFile(filepath.Join("scripts", e.Name()))
+		// embed.FS always uses forward slashes regardless of OS
+		data, err := fs.ReadFile(path.Join("scripts", e.Name()))
 		if err != nil {
 			return err
 		}
@@ -298,13 +300,14 @@ func extractDir(fs interface{ ReadDir(string) ([]os.DirEntry, error); ReadFile(s
 	if err != nil {
 		return err
 	}
-	dst := filepath.Join(dstBase, src)
+	dst := filepath.Join(dstBase, filepath.FromSlash(src))
 	if err := os.MkdirAll(dst, 0755); err != nil {
 		return err
 	}
 	for _, e := range entries {
-		srcPath := filepath.Join(src, e.Name())
-		dstPath := filepath.Join(dstBase, srcPath)
+		// embed.FS always uses forward slashes regardless of OS
+		srcPath := path.Join(src, e.Name())
+		dstPath := filepath.Join(dstBase, filepath.FromSlash(srcPath))
 		if e.IsDir() {
 			if err := extractDir(fs, srcPath, dstBase); err != nil {
 				return err