Adds support for reading OPENAI_API_KEY (and other variables) from a
user‑wide dotenv file (~/.codex.config). Precedence order is now:
1. explicit environment variable
2. project‑local .env (loaded earlier)
3. ~/.codex.config
Also adds a regression test that ensures the multiline editor correctly
handles cases where printable text and the CSI‑u Shift+Enter sequence
arrive in the same input chunk.
House‑kept with Prettier; removed stray temp.json artifact.
