Move GitHub token server-side & add ETag conditional caching to reduce API calls

[mukeshdhadhariya]

Summary

• Remove client-side token access; accept token via constructor or setAuthToken() (server-side only). Update README with instantiation instructions: new GitHubService(process.env.GITHUB_TOKEN).
• Add ETag / If-None-Match support and store ETags in cache meta. Conditional GET reduces API usage and avoids hitting rate limits.
• fetchWithRetry now accepts custom headers and handles 304 Not Modified responses. Cache now stores { data, etags, lastUpdated }.

Why

Exposing tokens in client-side code risks leaking credentials and causing abuse. Using conditional requests via ETag reduces bandwidth and GitHub API rate consumption.

Developer notes

• Instantiate server-side with: const svc = new GitHubService(process.env.GITHUB_TOKEN);
• Do not set tokens on window in production.
• Cache meta now includes ETags (keyed by endpoint) and lastUpdated for freshness checks.

[github-actions]

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[github-actions]

👤 Issue assigned to @mukeshdhadhariya.

[mukeshdhadhariya]

I have already opened a pull request addressing this issue. Please review and merge PR #892