feat: add hard delete flag and opal clean command

Add --hard flag to `opal delete` for permanent removal and a new
`opal clean` command to bulk-purge soft-deleted tasks with optional
--older duration filter.

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

opal-task/internal/engine/task.go

@@ -736,6 +736,48 @@ func (t *Task) IsRecurringInstance() bool {
 	return t.ParentUUID != nil
 }
 
+// GetDeletedTasks retrieves soft-deleted tasks, optionally filtered by age.
+// If olderThan is non-nil, only returns tasks deleted more than that duration ago.
+func GetDeletedTasks(olderThan *time.Duration) ([]*Task, error) {
+	db := GetDB()
+	if db == nil {
+		return nil, fmt.Errorf("database not initialized")
+	}
+
+	query := `
+		SELECT id, uuid, status, description, project, priority,
+		       created, modified, start, end, due, scheduled, wait, until_date,
+		       recurrence_duration, parent_uuid, annotations
+		FROM tasks
+		WHERE status = ?`
+	args := []interface{}{byte(StatusDeleted)}
+
+	if olderThan != nil {
+		cutoff := timeNow().Add(-*olderThan).Unix()
+		query += ` AND end < ?`
+		args = append(args, cutoff)
+	}
+
+	query += ` ORDER BY end ASC`
+
+	rows, err := db.Query(query, args...)
+	if err != nil {
+		return nil, fmt.Errorf("failed to query deleted tasks: %w", err)
+	}
+	defer rows.Close()
+
+	var tasks []*Task
+	for rows.Next() {
+		task, err := scanTask(rows)
+		if err != nil {
+			return nil, fmt.Errorf("failed to scan task: %w", err)
+		}
+		tasks = append(tasks, task)
+	}
+
+	return tasks, nil
+}
+
 // PopulateUrgency computes and sets the Urgency field on the given tasks.
 func PopulateUrgency(tasks ...*Task) {
 	cfg, _ := GetConfig()