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/stop.go

@@ -43,6 +43,10 @@ func stopTasks(args []string) error {
 		}
 	}
 
+	if len(tasks) == 0 {
+		return fmt.Errorf("no tasks matched filter")
+	}
+
 	for _, task := range tasks {
 		task.StopTask()
 		fmt.Printf("Stopped task: %s\n", task.Description)