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/root.go

@@ -32,7 +32,7 @@ var (
 
 // Command classification
 var commandNames = []string{
-	"add", "done", "modify", "delete",
+	"add", "done", "modify", "delete", "clean",
 	"start", "stop", "count", "projects", "tags",
 	"info", "edit", "server", "sync", "reports", "setup",
 	"version", "annotate", "denotate", "undo", "uncomplete", "log", "completion",
@@ -284,6 +284,7 @@ func init() {
 	undoCmd.GroupID = "task"
 	uncompleteCmd.GroupID = "task"
 	logCmd.GroupID = "task"
+	cleanCmd.GroupID = "task"
 
 	rootCmd.AddCommand(addCmd)
 	rootCmd.AddCommand(doneCmd)
@@ -298,6 +299,7 @@ func init() {
 	rootCmd.AddCommand(undoCmd)
 	rootCmd.AddCommand(uncompleteCmd)
 	rootCmd.AddCommand(logCmd)
+	rootCmd.AddCommand(cleanCmd)
 
 	// Other commands
 	countCmd.GroupID = "other"