fix: IMP-4/5/6 — parser allowlist, delete ID resolution, consistent errors

IMP-5: Replace strings.Contains(arg, ":") heuristic with an allowlist
of recognized attribute keys (ValidAttributeKeys). Colons in task
descriptions (URLs, "Meeting: topic") are no longer misinterpreted as
modifiers. Canonical key sets live in engine/keys.go and are shared
across parseAddArgs, ParseFilter, and ParseModifier. ParseModifier now
errors on unknown keys.

IMP-4: delete command now loads the working set and resolves display IDs
via GetTaskByDisplayID, matching the pattern used by done/modify.

IMP-6: All action commands (done, delete, modify, start, stop) now
return an error on no-match (stderr, exit 1). Previously done/delete
printed to stdout and exited 0; start/stop had no check at all.

Also adds requirements and design docs for the CLI UX improvements.

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

opal-task/cmd/delete.go

@@ -26,14 +26,31 @@ func deleteTasks(args []string) error {
 		return err
 	}
 
-	tasks, err := engine.GetTasks(filter)
+	// Load working set to resolve display IDs (matches done/modify pattern)
+	ws, err := engine.LoadWorkingSet()
 	if err != nil {
-		return err
+		return fmt.Errorf("failed to load working set: %w", err)
+	}
+
+	var tasks []*engine.Task
+
+	if len(filter.IDs) > 0 {
+		for _, id := range filter.IDs {
+			task, err := ws.GetTaskByDisplayID(id)
+			if err != nil {
+				return err
+			}
+			tasks = append(tasks, task)
+		}
+	} else {
+		tasks, err = engine.GetTasks(filter)
+		if err != nil {
+			return err
+		}
 	}
 
 	if len(tasks) == 0 {
-		fmt.Println("No tasks matched.")
-		return nil
+		return fmt.Errorf("no tasks matched filter")
 	}
 
 	fmt.Printf("Delete %d task(s)? (y/N): ", len(tasks))