Add Codex CLI plugin support

[wayyoungboy]

Summary

• add a Codex plugin manifest for memory-powermem while keeping Claude Code behavior intact
• rename the shared plugin root to apps/agent-plugin so Claude Code and Codex CLI use a neutral directory
• make installed-plugin init/status/stop/reset skills work for Claude Code and Codex CLI by resolving a shared plugin root
• update Codex setup docs to use codex plugin plus explicit codex mcp add, and refresh marketplace descriptions

Validation

• Codex plugin manifest validation for apps/agent-plugin
• JSON validation for marketplace and plugin manifest files
• workflow YAML parse check
• git diff --check
• sh -n for plugin shell scripts
• make -n package-agent-plugin and compatibility alias make -n package-claude-plugin
• temporary HOME Codex CLI install from the local checkout