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/import.go

@@ -3,12 +3,13 @@ package main
 import (
 	"bufio"
 	"bytes"
+	"cmp"
 	"encoding/json"
 	"fmt"
 	"os"
 	"os/exec"
 	"path/filepath"
-	"sort"
+	"slices"
 	"strings"
 	"time"
 
@@ -359,8 +360,8 @@ NOTE: Import requires direct database access and does not work with daemon mode.
 			for oldID, newID := range result.IDMapping {
 				mappings = append(mappings, mapping{oldID, newID})
 			}
-			sort.Slice(mappings, func(i, j int) bool {
-				return mappings[i].oldID < mappings[j].oldID
+			slices.SortFunc(mappings, func(a, b mapping) int {
+				return cmp.Compare(a.oldID, b.oldID)
 			})
 
 			fmt.Fprintf(os.Stderr, "Remappings:\n")