feat: improve CLI output with relative dates, rich feedback, and recurring task info

Add relative date formatting (today, tomorrow, in 3d, etc.) for list and
detail views. Add structured feedback helpers for add/complete/delete
operations showing display IDs and parsed modifiers. Change Complete() to
return spawned recurring instance so callers can display recurrence info.
Add AppendTask to working set for immediate display ID assignment.

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

opal-task/cmd/done.go

@@ -66,9 +66,9 @@ func completeTasks(args []string) error {
 		return fmt.Errorf("no tasks matched filter")
 	}
 
-	// Confirm if multiple tasks
 	if len(tasks) > 1 {
-		fmt.Printf("About to complete %d tasks. Proceed? (y/N): ", len(tasks))
+		fmt.Print(engine.FormatTaskConfirmList("complete", tasks, ws))
+		fmt.Printf("Proceed? (y/N): ")
 		var confirm string
 		fmt.Scanln(&confirm)
 		if confirm != "y" && confirm != "Y" {
@@ -80,14 +80,18 @@ func completeTasks(args []string) error {
 	// Complete tasks
 	completed := 0
 	for _, task := range tasks {
-		if err := task.Complete(); err != nil {
+		if _, err := task.Complete(); err != nil {
 			fmt.Fprintf(os.Stderr, "Warning: failed to complete task %s: %v\n", task.UUID, err)
 		} else {
 			completed++
 		}
 	}
 
-	fmt.Printf("Completed %d task(s).\n", completed)
+	if len(tasks) == 1 {
+		fmt.Printf("Completed task %s\n", engine.FormatTaskSummary(tasks[0], ws))
+	} else {
+		fmt.Printf("Completed %d task(s).\n", completed)
+	}
 
 	return nil
 }