Add environment variable contribution guide

[edehvalentine236-jpg]

Closes #522
Closes #526
Closes #524
Closes #520

Summary

This PR adds a contributor-facing guide for adding new client environment variables in Access Layer Client.

Changes

• Adds docs/environment-variables.md.
• Documents the expected declaration flow through .env.example.
• Explains how to add runtime validation in src/utils/env.utils.ts using the existing Zod schema pattern.
• Shows how to pass raw import.meta.env values into the central envSchema.parse(...) call.
• Provides the recommended application access pattern through the validated env export.
• Calls out direct import.meta.env usage in components, hooks, services, and utilities as an anti-pattern.
• Adds examples for required strings, optional strings, numeric values, and boolean feature flags.
• Includes a short checklist contributors can use before opening PRs that introduce new environment variables.

Testing

• Documentation-only change.
• Verified the uploaded file by reading it back from the docs/env-vars-522 branch through the GitHub API.
• Not run locally because Git is not available in this shell session, so I used the GitHub API workflow instead of a local checkout.

[drips-wave]

@edehvalentine236-jpg Great news! 🎉 Based on an automated assessment of this PR, the linked Wave issue(s) no longer count against your application limits.

You can now already apply to more issues while waiting for a review of this PR. Keep up the great work! 🚀

Learn more about application limits