refactor(cmd): migrate sort.Slice to slices.SortFunc (bd-u2sc.2)

Modernize sorting code to use Go 1.21+ slices package:
- Replace sort.Slice with slices.SortFunc across 16 files
- Use cmp.Compare for orderable types (strings, ints)
- Use time.Time.Compare for time comparisons
- Use cmp.Or for multi-field sorting
- Use slices.SortStableFunc where stability matters

Benefits: cleaner 3-way comparison, slightly better performance,
modern idiomatic Go.

Part of GH#692 refactoring epic.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

cmd/bd/doctor.go

@@ -7,7 +7,7 @@ import (
 	"fmt"
 	"os"
 	"path/filepath"
-	"sort"
+	"slices"
 	"strings"
 	"time"
 
@@ -895,15 +895,15 @@ func printDiagnostics(result doctorResult) {
 		fmt.Println(ui.RenderWarn(ui.IconWarn + "  WARNINGS"))
 
 		// Sort by severity: errors first, then warnings
-		sort.Slice(warnings, func(i, j int) bool {
+		slices.SortStableFunc(warnings, func(a, b doctorCheck) int {
 			// Errors (statusError) come before warnings (statusWarning)
-			if warnings[i].Status == statusError && warnings[j].Status != statusError {
-				return true
+			if a.Status == statusError && b.Status != statusError {
+				return -1
 			}
-			if warnings[i].Status != statusError && warnings[j].Status == statusError {
-				return false
+			if a.Status != statusError && b.Status == statusError {
+				return 1
 			}
-			return false // maintain original order within same severity
+			return 0 // maintain original order within same severity
 		})
 
 		for i, check := range warnings {