feat: add todo function

[jjlai5]

Goals

• Enable users to create and manage todo items via CLI.
• Provide persistent local storage of todos.
• Support basic task management operations: add, list, mark as done, delete.

User Stories

• As a developer, I want to quickly add a note to myself without leaving the terminal.
• As a contributor, I want to list unfinished tasks before submitting a patch.

CLI Commands

codex todo add "Fix bug when input is null"
codex todo list

Sample JSON structure (.codex/todo.json)

[
  {
    "id": 1,
    "task": "Fix bug when input is null",
    "done": false,
    "created_at": "2025-05-26T22:00:00Z"
  },
  {
    "id": 2,
    "task": "Refactor utils module",
    "done": true,
    "created_at": "2025-05-26T22:05:00Z"
  }
]

File Structure

.codex/
  └── todo.json

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