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/internal/engine/task.go

@@ -694,25 +694,27 @@ func (t *Task) GetTags() ([]string, error) {
 	return tags, nil
 }
 
-// Complete marks a task as completed
-func (t *Task) Complete() error {
+// Complete marks a task as completed.
+// Returns the next recurring instance if one was spawned, or nil.
+func (t *Task) Complete() (*Task, error) {
 	t.Status = StatusCompleted
 	now := timeNow()
 	t.End = &now
 
 	if err := t.Save(); err != nil {
-		return err
+		return nil, err
 	}
 
 	// If this is a recurring instance, spawn next instance
 	if t.ParentUUID != nil {
-		if err := SpawnNextInstance(t); err != nil {
-			// Log error but don't fail the completion
-			return fmt.Errorf("completed task but failed to spawn next instance: %w", err)
+		next, err := SpawnNextInstance(t)
+		if err != nil {
+			return nil, fmt.Errorf("completed task but failed to spawn next instance: %w", err)
 		}
+		return next, nil
 	}
 
-	return nil
+	return nil, nil
 }
 
 // Delete marks a task as deleted (soft delete)