[codex] add CDP extension transport

[sleepyy-dog]

Summary

• add an optional Manifest V3 CDP extension transport for web-access
• route the local CDP API through the extension on the existing 127.0.0.1:3456 API surface
• keep native remote-debugging CDP as a fallback via check-deps.mjs
• add tests for the extension bridge HTTP/WebSocket behavior and repository structure

Validation

• node --check scripts/check-deps.mjs scripts/cdp-proxy.mjs scripts/check-cdp.mjs scripts/cdp-extension-transport.mjs extension/background.js
• node --test tests/*.test.mjs
• git diff --cached --check