feat: add script output tests and CI coverage

[nholthaus]

Summary

• add CTest-based script output coverage for helloWorld, nestedFor, and multi-variable echo
• move argument dereferencing into the commands that consume expanded values
• add GitHub Actions CI and publish build/test output in PR summaries and artifacts

Testing

• CCACHE_TEMPDIR=/tmp/ccache-tmp cmake --build --preset build-debug
• ctest --test-dir build/linux-ninja-debug --output-on-failure

[github-actions]

Test Results

13 tests   13 ✅  0s ⏱️
 1 suites   0 💤
 1 files     0 ❌

Results for commit dd1160b.

♻️ This comment has been updated with latest results.