Exclude link-checker config from the packaged gem (STF-557)

[oschwald]

Follow-up to the merged STF-557 PR: the gemspec packages Dir['**/*'] minus an exclusion list that didn't cover the link-checking config added in that PR, so lychee.toml/mise.toml/mise.lock (and .gitignore) were shipping in the published gem. Add them to the exclusion list.

This addresses @horgh's review note; the original STF-557 PR had already merged, so this is a separate PR.

Part of STF-557.

🤖 Generated with Claude Code

[gemini-code-assist]

Code Review

This pull request updates the gemspec to exclude configuration and lock files like .gitignore, lychee.toml, mise.lock, and mise.toml from the packaged gem. The reviewer suggested a best-practice improvement to use git ls-files instead of Dir['**/*'] to avoid packaging untracked local files.

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[gemini-code-assist]

Using Dir['**/*'] to determine the files to package in the gem is risky because it will include any untracked local files, temporary test databases, or potentially sensitive credentials/configuration files present in the working directory when gem build is run.

Using git ls-files is the standard best practice for Ruby gems. It ensures only tracked files are packaged, while falling back to Dir['**/*'] if git is unavailable (e.g., when building from a source tarball).

  s.files       = begin
                    IO.popen(%w[git ls-files -z], chdir: __dir__, err: IO::NULL, &:read).split("\x0")
                  rescue StandardError
                    Dir['**/*']
                  end.difference(Dir['.github/**/*', '.gitignore', 'lychee.toml', 'mise.lock', 'mise.toml', 'dev-bin/**/*', 'test/**/*', 'CLAUDE.md', 'Gemfile*', 'Rakefile', '*.gemspec', 'README.dev.md'])