chore: Make GitHub fetch counts configurable via env vars

[O2sa]

The numbers used when querying GitHub (repository count, pull request count, issue count, discussion count) are currently hardcoded in lib/github.ts. These should be configurable via runtime environment variables with sensible fallbacks so deployments can tune API usage without code changes.

Suggested env vars and defaults:

• GITHUB_REPO_COUNT (default: 30)
• GITHUB_PR_COUNT (default: 80)
• GITHUB_ISSUE_COUNT (default: 20)
• GITHUB_DISCUSSION_COUNT (default: 10)

Tasks

• Update lib/github.ts to read the above env vars at runtime and parse them to integers. Use the listed defaults when values are missing or invalid.
• Replace the hardcoded numbers in fetchUserDataFromGitHub variables/GraphQL call arguments (repoCount, prCount, issueCount, discussionCount) with the parsed values.
• Add validation to ensure the values are positive integers and apply reasonable upper bounds to avoid accidental large queries.
• Document the new env vars in .env.example and README.md (or relevant docs).
• Add unit tests for the parsing/validation helper (if applicable) to ensure fallbacks and invalid values are handled.

Notes

• These env vars should be read from the server/runtime environment (not client-side public vars). Keep existing defaults equal to the current hardcoded values to avoid behavior changes.

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