docs: document environment configuration

[felixvippp-ai]

Closes #54

Summary

• add a safe committed .env.example covering every environment variable currently read by the app
• unignore .env.example while keeping real .env and .env.* files ignored
• add a README Configuration section for PORT, CORS_ALLOWED_ORIGINS, and NODE_ENV

Documented variables

• PORT - HTTP listen port, default 3001
• CORS_ALLOWED_ORIGINS - comma-separated exact Origin allowlist, default empty
• NODE_ENV - runtime mode; test skips rate limiting and suppresses duration logs

Validation

• npm.cmd run build
• npm.cmd run lint
• $env:NODE_ENV='test'; node --test dist/*.test.js dist/**/*.test.js - 69 tests passed
• git check-ignore -v .env.example shows the explicit !.env.example unignore rule
• git add --dry-run .env.example confirms the example file is tracked normally
• rg process.env / src README package.json .env.example confirmed the documented variables match current env reads

[mikewheeleer]

thanks for the effort here @felixvippp-ai! 🙏 issue #54 isn't assigned to you, and we merge from the assigned contributor to keep the campaign fair. please claim an open unassigned issue first, then open your PR. closing this one for now — hope to see it back 🙌