fix(config): improve cross-platform path handling and regex efficiency

Code review fixes for bd config validate:
- Use filepath.Join instead of string concatenation for paths
- Use filepath.Dir instead of strings.LastIndex for parent directory
- Move gitSSHRemotePattern regex to package-level var (compile once)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

cmd/bd/config.go

@@ -3,6 +3,7 @@ package main
 import (
 	"fmt"
 	"os"
+	"path/filepath"
 	"regexp"
 	"sort"
 	"strings"
@@ -14,6 +15,9 @@ import (
 	"github.com/steveyegge/beads/internal/syncbranch"
 )
 
+// gitSSHRemotePattern matches standard git SSH remote URLs (user@host:path)
+var gitSSHRemotePattern = regexp.MustCompile(`^[a-zA-Z0-9._-]+@[a-zA-Z0-9][a-zA-Z0-9._-]*:.+$`)
+
 var configCmd = &cobra.Command{
 	Use:     "config",
 	GroupID: "setup",
@@ -347,7 +351,7 @@ func validateSyncConfig(repoPath string) []string {
 	var issues []string
 
 	// Load config.yaml directly from the repo path
-	configPath := repoPath + "/.beads/config.yaml"
+	configPath := filepath.Join(repoPath, ".beads", "config.yaml")
 	v := viper.New()
 	v.SetConfigType("yaml")
 	v.SetConfigFile(configPath)
@@ -433,22 +437,19 @@ func isValidRemoteURL(url string) bool {
 	}
 
 	// Also allow standard git remote patterns (user@host:path)
-	// The host must have at least one character before the colon
-	// Pattern: username@hostname:path where hostname has at least 2 chars
-	gitSSHPattern := regexp.MustCompile(`^[a-zA-Z0-9._-]+@[a-zA-Z0-9][a-zA-Z0-9._-]*:.+$`)
-	return gitSSHPattern.MatchString(url)
+	return gitSSHRemotePattern.MatchString(url)
 }
 
 // findBeadsRepoRoot walks up from the given path to find the repo root (containing .beads)
 func findBeadsRepoRoot(startPath string) string {
 	path := startPath
 	for {
-		beadsDir := path + "/.beads"
+		beadsDir := filepath.Join(path, ".beads")
 		if info, err := os.Stat(beadsDir); err == nil && info.IsDir() {
 			return path
 		}
-		parent := path[:strings.LastIndex(path, "/")]
-		if parent == path || parent == "" {
+		parent := filepath.Dir(path)
+		if parent == path {
 			return ""
 		}
 		path = parent