Use GITHUB_TOKEN instead of custom infra token in workflows

[Aditya30ag]

Summary

This PR removes the dependency on the custom infra_automation secret by switching workflows to use GitHub’s built-in GITHUB_TOKEN.

Changes Made

• Replaced secrets.infra_automation with ${{ secrets.GITHUB_TOKEN }}
• Added explicit workflow-level permissions for required access
• Updated both:
  • test_and_deploy.yml
  • issue_assignment.yml

Why this change

• Prevents workflow failures when the custom secret is missing
• Aligns with GitHub Actions best practices
• Reduces maintenance overhead and hidden dependencies

Impact

No functional change to behavior — workflows continue to operate with safer and more reliable authentication.
#79