fix(actions): support GITHUB_PAT in duplicate issue scanner

[mohdsubhan1756]

Description

Fixes #3350

This PR updates check-duplicates.js to support GITHUB_PAT as a fallback authentication token when GITHUB_TOKEN is not available.

Previously, the script only checked for GITHUB_TOKEN, causing it to fail even when a valid GITHUB_PAT was configured. This behavior was inconsistent with the project documentation and other GitHub integrations in the codebase.

Changes

• Add support for GITHUB_PAT as a fallback token.
• Maintain backward compatibility with existing GITHUB_TOKEN configurations.
• Update validation error messages to indicate support for both environment variables.
• Align implementation with documented authentication requirements.

Pillar

• 🎨 Pillar 1 — New Theme Design
• 📐 Pillar 2 — Geometric SVG Improvement
• 🕐 Pillar 3 — Timezone Logic Optimization
• 🛠️ Other (Bug fix, refactoring, docs)

Visual Preview

N/A (GitHub Actions / backend script change)

Checklist before requesting a review:

• I have read the CONTRIBUTING.md file.
• I have tested these changes locally (localhost:3000/api/streak?user=YOUR_USERNAME).
• I have run npm run format and npm run lint locally and resolved all errors (CI will fail otherwise).
• My commits follow the Conventional Commits format (e.g., feat(themes): ..., fix(calculate): ...).
• I have updated README.md if I added a new theme or URL parameter.
• I have started the repo.
• I have made sure that i have only one commit to merge in this PR.
• The SVG output matches the CommitPulse "premium quality" aesthetic standard (no raw elements, smooth animations, correct fonts).
• (Recommended) I joined the CommitPulse Discord community for contributor discussions, mentorship, and faster PR support.

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[github-actions]

👋 Hey @mohdsubhan1756, welcome to CommitPulse! 🎉

Thanks for opening your first pull request — this is a big deal and we appreciate the effort!

While you wait for a review, please double-check:

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[github-actions]

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