[codex] fix strict GitHub username validation

[saurabhhhcodes]

Description\n\nTighten GitHub username validation so it matches GitHub's 39-character limit and rejects invalid leading/trailing hyphen patterns.\n\n## Validation\n\n- npm test -- --run lib/validations.accessibility.test.ts lib/validations.githubParamsSchema.test.ts lib/validations.streakParamsSchema.test.ts lib/validations.massive-scaling.test.ts\n- git diff --check

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[github-actions]

👋 Hey @saurabhhhcodes! Thanks for your contribution! 🎉

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[Aamod-Dev]

Solid validation fix! I went through the changes and ensuring GitHub username validations accurately match GitHub's internal regex prevents rejected input for valid usernames.
Labels applied:

• level:intermediate: Regex parsing validation.
• quality:clean: Accurate boundary adjustments.
• ype:bug: Fixes input routing.