feat: add NuGet package support

[openaitx-system]

Summary

Add NuGet package publishing infrastructure for MiniPdf.

Changes

• MiniPdf.csproj: Added complete NuGet metadata (Authors, RepositoryUrl, RepositoryType, PackageProjectUrl, PackageReadmeFile)
• nuget-publish.yml: New GitHub Actions workflow that automatically publishes to NuGet.org when a GitHub Release is created
• ci.yml: Added dotnet pack verification step so every CI build ensures the package can be packed successfully
• README.md: Added dotnet add package MiniPdf install instructions

How to publish a new version

1. Update <Version> in src/MiniPdf/MiniPdf.csproj
2. Create a GitHub Release with a tag like v0.2.0
3. The nuget-publish.yml workflow will automatically pack and push to NuGet.org

Required secret

• NUGET_API_KEY — NuGet.org API key with push permission for the MiniPdf package

Closes #5

[Copilot]

Pull request overview

Adds NuGet packaging and publishing automation for the MiniPdf library, enabling release-driven publishing to NuGet.org and CI verification that the project can be packed.

Changes:

• Added NuGet package metadata (authors, repo URLs, readme) to MiniPdf.csproj and included the root README.md in the package.
• Introduced a nuget-publish.yml workflow to pack and push on GitHub Release publish.
• Updated CI to run dotnet pack as a packaging sanity check, and updated README with NuGet install instructions.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

File	Description
src/MiniPdf/MiniPdf.csproj	Adds NuGet metadata and attempts to include the repository README in the produced package.
README.md	Documents installing MiniPdf from NuGet.
.github/workflows/nuget-publish.yml	Implements release-triggered pack + publish to NuGet.org.
.github/workflows/ci.yml	Adds a dotnet pack step to ensure packaging succeeds in CI.

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[Copilot]

PackagePath="\" uses a Windows-style separator and can end up creating an unexpected folder name inside the .nupkg on non-Windows platforms. For packing the README at the package root (to match PackageReadmeFile), use an empty PackagePath (or /) instead.

Suggested change

	<None Include="..\..\README.md" Pack="true" PackagePath="\" />
	<None Include="..\..\README.md" Pack="true" PackagePath="" />

[Copilot]

The workflow sets PackageVersion from the Git tag, but the PR description instructs maintainers to update <Version> in MiniPdf.csproj. As-is, those can diverge (CI packing uses the csproj version, while publishing uses the tag). Consider either (1) relying on the csproj <Version> and dropping the -p:PackageVersion=... override, or (2) adding a validation step that fails if the tag version doesn't match the csproj version, and update the publish instructions accordingly.

[Copilot]

VERSION=${GITHUB_REF_NAME#v} assumes the tag is prefixed with v and doesn't validate the result. If the release tag is missing the prefix (or is just v), the version passed to dotnet pack can become invalid/empty and fail in a confusing way. Consider adding a small validation step to assert the extracted version is non-empty and matches an expected NuGet/SemVer pattern before packing.

Suggested change

	run: echo "VERSION=${GITHUB_REF_NAME#v}" >> $GITHUB_OUTPUT
	run: |
	RAW_TAG="${GITHUB_REF_NAME}"
	VERSION="${RAW_TAG#v}"
	if [ -z "$VERSION" ]; then
	echo "Error: Extracted version is empty. Ensure the release tag is of the form 'v<version>' (e.g., 'v1.2.3')."
	exit 1
	fi
	if ! echo "$VERSION" | grep -Eq '^[0-9]+\.[0-9]+\.[0-9]+(-[0-9A-Za-z.-]+)?$'; then
	echo "Error: Extracted version '$VERSION' is not a valid SemVer/NuGet version (expected format: MAJOR.MINOR.PATCH[-PRERELEASE])."
	exit 1
	fi
	echo "Using version: $VERSION"
	echo "VERSION=$VERSION" >> "$GITHUB_OUTPUT"