Add PR validation for AoC submissions

[Kavignon]

I must ensure that all pull requests for AoC challenge solutions follow a consistent format and include the required files. This issue proposes adding a validation step to our CI pipeline that checks for the following.

What I'm thinking about:

• Branch naming:
  • The branch must follow the format aoc/YYYY_day_*.
• Required files: For a given day_* challenge, the following files must be included:
  • lib/year2024/day<day_number>.ex (Solution script)
  • test/year2024/day<day_number>_test.exs (Test suite)
  • inputs/2024/day_<day_number>_part_1.txt (Input file, Part 1)
  • inputs/2024/day_<day_number>_part_2.txt (Input file, Part 2)
  • notebooks/2024_day_<day_number>.livemd (Livebook notebook for the day)
  • bench/2024_day_<day_number>_bench.exs (Benchmarks file)
• Update the master index file for the year (e.g., notebooks/2024_index.livemd).
• Validation Script:
  • Create a Mix task (mix check_pr) to verify that the required files are present for a given PR.
• Integration with CI:
  • Add a validate-pr job to the existing GitHub Actions workflow.
• Ensure that the validation job runs on all PRs targeting main.
• Failure Criteria:
  • The CI job fails if required files are missing or the branch name does not follow the required format.
  • Provide a clear list of missing files in the CI output.