Add CI workflow and NuGet publishing automation

[xqiu]

Summary

• add a GitHub Actions CI workflow that restores, builds, tests, and packs the solution on pushes and pull requests
• introduce a workflow for manually publishing the NuGet package to NuGet.org using a stored API key
• provide cross-platform scripts and a solution file to support packing and pushing the package locally

Testing

• not run (dotnet CLI unavailable in container)

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Codex Task

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Ship real get-pip bootstrapper

The Init flow copies get-pip.py next to the library, runs it to install pip, and then creates the virtual environment. The committed get-pip.py is only print("get-pip placeholder"), so executing it never installs pip. As a result the new virtual environment will be created without pip and InstallRequirement will reliably fail (python -m pip install exits non‑zero) whenever dependencies are installed. This file needs to contain the actual get‑pip bootstrap script or the code should use another supported mechanism such as python -m ensurepip.

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