chore: extract the SDK into its own repository

Standalone home for the CertifiEd .NET client SDK, split out of the
platform repository with its history.

- src/ layout with its own solution, Directory.Build.props and
  global.json (warnings as errors, XML docs required).
- Gitea Actions CI: build and pack on every push, publish to nuget.org
  on a v* tag using the NUGET_API_KEY secret.
- CONTRIBUTING documents the release flow and the one-time CI setup.

Directory.Build.props

@@ -0,0 +1,19 @@
+<Project>
+
+  <PropertyGroup>
+    <TargetFramework>net10.0</TargetFramework>
+    <LangVersion>latest</LangVersion>
+    <Nullable>enable</Nullable>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
+    <WarningsAsErrors />
+    <EnforceCodeStyleInBuild>true</EnforceCodeStyleInBuild>
+    <AnalysisLevel>latest-recommended</AnalysisLevel>
+  </PropertyGroup>
+
+  <!-- Reproducible builds on CI. -->
+  <PropertyGroup Condition="'$(GITHUB_ACTIONS)' == 'true' or '$(CI)' == 'true'">
+    <ContinuousIntegrationBuild>true</ContinuousIntegrationBuild>
+  </PropertyGroup>
+
+</Project>