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/cmd/clean.go

@@ -0,0 +1,77 @@
+package cmd
+
+import (
+	"fmt"
+	"os"
+	"time"
+
+	"git.jnss.me/joakim/opal/internal/engine"
+	"github.com/spf13/cobra"
+)
+
+var olderFlag string
+
+var cleanCmd = &cobra.Command{
+	Use:   "clean",
+	Short: "Purge soft-deleted tasks from the database",
+	Run: func(cmd *cobra.Command, args []string) {
+		if err := cleanTasks(); err != nil {
+			fmt.Fprintf(os.Stderr, "Error: %v\n", err)
+			os.Exit(1)
+		}
+	},
+}
+
+func init() {
+	cleanCmd.Flags().StringVar(&olderFlag, "older", "", "Only purge tasks deleted longer than this duration ago (e.g. 30d, 1w)")
+}
+
+func cleanTasks() error {
+	var olderThan *time.Duration
+	if olderFlag != "" {
+		d, err := engine.ParseRecurrencePattern(olderFlag)
+		if err != nil {
+			return fmt.Errorf("invalid duration %q: %w", olderFlag, err)
+		}
+		olderThan = &d
+	}
+
+	tasks, err := engine.GetDeletedTasks(olderThan)
+	if err != nil {
+		return err
+	}
+
+	if len(tasks) == 0 {
+		fmt.Println("No deleted tasks to purge.")
+		return nil
+	}
+
+	if dryRunFlag {
+		fmt.Printf("Would permanently remove %d deleted task(s).\n", len(tasks))
+		return nil
+	}
+
+	if len(tasks) > 1 {
+		fmt.Printf("Permanently remove %d deleted task(s)? This cannot be undone. (y/N): ", len(tasks))
+		var confirm string
+		fmt.Scanln(&confirm)
+		if confirm != "y" && confirm != "Y" {
+			fmt.Println("Cancelled.")
+			return nil
+		}
+	}
+
+	for _, task := range tasks {
+		if err := task.Delete(true); err != nil {
+			return fmt.Errorf("failed to purge task %s: %w", task.UUID, err)
+		}
+	}
+
+	fmt.Printf("Purged %d deleted task(s).\n", len(tasks))
+
+	if err := engine.CleanupChangeLog(); err != nil {
+		fmt.Fprintf(os.Stderr, "Warning: failed to clean up change log: %v\n", err)
+	}
+
+	return nil
+}