feat: add reusable .NET build & test workflow

Co-Authored-By: Claude Opus 5.5 <[email protected]>

README.md

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+# dotnet-build-test
+
+Reusable Gitea Actions workflow: `dotnet restore` → `build` → `test`, with no Docker. Meant as a PR gate, for example so Renovate can automerge.
+
+## Usage
+
+```yaml
+on:
+  pull_request:
+
+jobs:
+  build-test:
+    uses: templates/dotnet-build-test/.gitea/workflows/dotnet-build-test.yaml@v1
+    with:
+      solution: MyApp.slnx
+      # dotnet-version: 10.0.x
+      # configuration: Release
+      # warnaserror: true
+      # working-directory: .
+      # test-filter: "Category!=Integration"
+```
+
+## Inputs
+
+| Name | Default | Description |
+|---|---|---|
+| `solution` | `""` | Solution/project file, relative to `working-directory`. If empty, dotnet uses the single sln/proj in that directory. |
+| `dotnet-version` | `10.0.x` | .NET SDK version |
+| `configuration` | `Release` | Build configuration |
+| `warnaserror` | `true` | Pass `-warnaserror` to `dotnet build` |
+| `working-directory` | `.` | Where dotnet commands run |
+| `test-filter` | `""` | Optional `dotnet test --filter` expression |
+
+No secrets.
+
+## Versioning
+
+Only major tags (`v1`, `v2`, …). Fixes and new optional inputs move the existing tag. Changing or removing inputs, or changing behavior, needs a new major tag.