Move markdown-unlit to build-tools

[derelbenkoenig]

Summary

This PR moves markdown-unlit from dependencies to build-tools in the package.yaml test configuration and removes the corresponding empty import from README.lhs.

Why this change?

When markdown-unlit is listed in dependencies, we need to import it in the Haskell code to avoid unused-package warnings from GHC. However, since markdown-unlit is only used as a preprocessor (via -pgmL markdown-unlit), we don't actually need to import anything from it in our code.

By moving it to build-tools instead, we:

• Avoid unused-package warnings without needing empty imports
• Make the dependency relationship clearer (it's a build tool, not a runtime dependency)
• Follow Stack/Cabal best practices for preprocessor dependencies

Changes

• Moved markdown-unlit from dependencies to build-tools in package.yaml
• Removed import Text.Markdown.Unlit () from README.lhs
• Regenerated any files via stack build --fast --test --no-run-tests

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