refactor: deduplicate engine internals, replace bubble sorts, remove dead code

Extract shared code that was duplicated across functions:
- taskJSON struct (MarshalJSON/UnmarshalJSON) to package-level type
- scanTask(scanner) helper for GetTask/GetTasks (~70 identical lines)
- monthNames map for parseMonthName/parseDayAndMonth
- applyNonDateAttribute helper for Apply/ApplyToNew
- resolveDisplayID calls replace inline loops in FormatTaskListWithFormat

Replace O(n²) bubble sorts with sort.Slice in all four report sort
functions (sortByUrgency, NewestReport, NextReport, OldestReport).

Remove dead code: formatTimeWithColor (unused, also used time.Now()
instead of timeNow()), getCurrentTimestamp (unnecessary wrapper).

Remove ~20 comments that restated the next line of code.

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

opal-task/internal/engine/display.go

@@ -29,15 +29,9 @@ func FormatTaskListWithFormat(tasks []*Task, ws *WorkingSet, format string) stri
 	if format == "minimal" {
 		result := ""
 		for i, task := range tasks {
-			displayID := i + 1
-			if ws != nil {
-				// Use working set display ID if available
-				for id, uuid := range ws.byID {
-					if uuid == task.UUID {
-						displayID = id
-						break
-					}
-				}
+			displayID := resolveDisplayID(task, ws)
+			if displayID == 0 {
+				displayID = i + 1
 			}
 			urgency := task.CalculateUrgency(coeffs)
 			urgencyColor := getUrgencyColor(urgency)
@@ -71,15 +65,9 @@ func FormatTaskListWithFormat(tasks []*Task, ws *WorkingSet, format string) stri
 
 	// Add rows
 	for i, task := range tasks {
-		displayID := i + 1
-		if ws != nil {
-			// Use working set display ID if available
-			for id, uuid := range ws.byID {
-				if uuid == task.UUID {
-					displayID = id
-					break
-				}
-			}
+		displayID := resolveDisplayID(task, ws)
+		if displayID == 0 {
+			displayID = i + 1
 		}
 
 		urgency := task.CalculateUrgency(coeffs)
@@ -270,8 +258,6 @@ func FormatTagCounts(tagCounts map[string]int) string {
 	return t.Render()
 }
 
-// Helper functions
-
 func formatStatus(status Status) string {
 	switch status {
 	case StatusPending:
@@ -322,7 +308,6 @@ func formatUrgency(urgency float64) string {
 }
 
 func getUrgencyColor(urgency float64) *color.Color {
-	// Returns color for minimal format
 	if urgency >= 10.0 {
 		return color.New(color.FgHiRed, color.Bold)
 	} else if urgency >= 5.0 {
@@ -362,14 +347,6 @@ func formatDue(due *time.Time) string {
 	return rel
 }
 
-func formatTimeWithColor(t time.Time) string {
-	now := time.Now()
-	if t.Before(now) {
-		return color.RedString(t.Format("2006-01-02 15:04"))
-	}
-	return t.Format("2006-01-02 15:04")
-}
-
 func formatTime(t time.Time) string {
 	return t.Format("2006-01-02 15:04")
 }