[codex] add first environment guide

[salim4n]

Closes #55

What changed

• Added a first user-facing environment authoring guide from a blank TypeScript project.
• Covers reset/seeding, observations, action spaces, stepping, named rewards, done results and a runnable smoke script.
• Added troubleshooting for invalid action, observation, reward and done shapes.
• Linked the guide from the main README.

Validation

• bun run ci