feat: add Codex integration

[bintangtimurlangit]

What changed

• adds a native Codex plugin manifest that reuses the existing figma-cli skill
• adds a Codex option to init-agent so it can write the existing AGENTS.md guidance without Cursor files
• adds the short Codex setup flow to the README

Testing

• npm test: 520 passed, 1 existing live test skipped
• Codex plugin validation passed
• skill validation passed
• tested init-agent with the Codex option in a clean temporary project
• installed the marketplace from this GitHub branch and confirmed Codex finds figma-cli 2.1.2