feat: 添加 mycodex 入口，支持 OpenAI Codex CLI

- 在 launcher 中注册 codex 后端，兼容 ~/.codex/sessions 日期嵌套目录

- 解析 session_meta.payload.cwd 过滤当前目录会话

- 添加 myagents codex 子命令与 mycodex 独立入口、补全及 Makefile 链接

- 更新 pyproject.toml 控制台脚本与 README 文档

- 补充 codex 相关单元测试，45 tests passed

scripts/rm_user_local_myagents.py

@@ -1,12 +1,12 @@
 #!/usr/bin/env python3
-"""Remove ~/.local/bin/{myagents,myclaude,mykimi} symlinks to this repo's venv."""
+"""Remove ~/.local/bin/{myagents,myclaude,mykimi,mycodex} symlinks to this repo's venv."""
 
 from __future__ import annotations
 
 import os
 import sys
 
-_COMMANDS = ("myagents", "myclaude", "mykimi")
+_COMMANDS = ("myagents", "myclaude", "mykimi", "mycodex")
 
 
 def _remove_link(link: str, want: str) -> bool: