fix: gate all todo prompt instructions behind hideTodoPromptInstructions

- Add hideTodoPromptInstructions to ReminderInstructionsProps and
  AgentUserMessageProps; thread it through getUserMessagePropsFromAgentProps
  and the reminderProps construction in AgentUserMessage
- Gate todo tool guidance and markdown fallback paragraphs in
  gpt5Prompt, gpt51Prompt, gpt52Prompt, gpt53CodexPrompt: suppress
  the full planning section when the background agent manages todos
- Restore the missing fallback paragraph for gpt51 and gpt52 that
  was accidentally removed (empty branch body)
- Gate the CRITICAL todo checklist and plan-tracking bullets in
  AlternateGPTPrompt (defaultAgentInstructions) using hasTodoTool
- Gate todo-specific reminder instructions in Gpt5ReminderInstructions