fix: show startup warning for missing GITHUB_TOKEN in production

[rishab11250]

Description

The validateEnv() function in src/server.js suppressed all startup warnings in production mode (NODE_ENV=production). This meant if GITHUB_TOKEN was not set, the app ran silently with only 60 unauthenticated API requests per hour instead of 5000 — users saw mysterious rate-limit errors with zero server-side diagnostics.

Change

Removed the !IS_PRODUCTION guard so critical environment warnings always display at startup, regardless of environment.

Before

if (!IS_PRODUCTION && warnings.length > 0) {

After

if (warnings.length > 0) {

Testing

• Ran npm test — all existing tests pass (3 pre-existing failures unrelated to this change)
• Tested manually: warning prints in both development and production when GITHUB_TOKEN is missing
• Tested manually: no warning prints when GITHUB_TOKEN is set

Closes #103

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