Exclude archived repositories from indexing

[Copilot]

Archived repositories cannot be modified, so indexing them provides no value. This change ensures they are filtered out at the repository listing stage and documents the behavior.

Changes

• main.go: fetchRepositories already skips archived repos via repo.GetArchived() — no code change needed
• README.md: Added "Archived Repositories" section documenting that archived repos are filtered out at the GitHub API listing stage and will not be indexed

Original prompt

Update this application to properly handle GitHub repositories that are archived. Archived repository should effectively be ignored. They cannot be modified, therefore treat them like they don't exist. When listing all of the repositories ideally filter them out at that stage so they are not even listed. If they cannot be filtered out at that stage then when they are retrieved and the details provided filter them out at that stage. Update the relevant docs to mention this behavior.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 0.00%. Comparing base (a741e0a) to head (5619588).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main     #54   +/-   ##
=====================================
  Coverage   0.00%   0.00%           
=====================================
  Files          1       1           
  Lines        750     750           
=====================================
  Misses       750     750           

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