Add the guides and wiki pages for the C and C++ and C# quality checks

Task: MIL-007#6

guides/quality-checks-csharp.md

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+# Reading C# quality-check results
+
+How a software engineer calls the shared C# quality checks, reads a failed run
+and makes it pass. The workflow checks the formatting and builds a solution or
+project with warnings as errors, on an exact .NET SDK of version 10 or later,
+changes no file and needs no secret. Its contract is ADR-0008 in this
+repository.
+
+## Calling the workflow
+
+```yaml
+name: Quality C#
+
+on:
+  pull_request:
+  push:
+    branches: [ main ]
+
+jobs:
+  quality:
+    permissions:
+      contents: read
+    uses: TirSystem/github-action/.gitea/workflows/quality-checks-csharp.yml@v1
+    with:
+      project: src/App.sln
+```
+
+| Input | Default | Meaning |
+| --- | --- | --- |
+| `working-directory` | `.` | Directory, relative to the repository root, whose files are checked |
+| `project` | empty | Solution or project file, relative to `working-directory`; empty uses the single `*.sln` or `*.slnx`, else the single `*.csproj`, that is tracked there |
+| `dotnet-sdk-version` | `10.0.401` | Exact version of the .NET SDK, 10 or later; a channel such as `10.0`, a range and a version below 10 are refused |
+| `tools-ref` | `v1` | Tag or branch of `TirSystem/github-action` from which the script `tools/quality-checks-csharp.sh` is fetched |
+
+Reference the workflow at the major tag (`@v1`) or at an exact tag, never at
+`main`. A call at an exact tag is pinned completely only when `tools-ref` names
+the same tag. See the upgrade guide. The SDK version is exact: raising the
+default is a new release of this repository, not a silent change.
+
+## What the repository needs
+
+- A solution or project that builds with the SDK. Several candidates need the
+  `project` input; C# files with no solution or project fail with
+  `[configuration]`.
+- The network: `dotnet format` and `dotnet build` restore the NuGet packages of
+  the project in the run.
+- Rules in `.editorconfig` (and the analyzers the project enables, for example
+  with `AnalysisLevel` and `EnforceCodeStyleInBuild`) decide what a finding is.
+
+## What is checked
+
+| Files | Tool | What it reports |
+| --- | --- | --- |
+| the solution or project | `dotnet format --verify-no-changes` | formatting and style that differ from `.editorconfig` |
+| the same | `dotnet build -warnaserror` | compiler and analyzer warnings, which fail the build |
+
+Untracked files are not checked. A repository without `*.cs` files passes with
+a notice. Project tests are not run.
+
+## Reading a run
+
+1. Open the status check on the pull request, then the job
+   `quality-checks-csharp`.
+2. Each tool has its own log group. Both run even when the first failed, so one
+   run shows all findings.
+3. A tool ends with `PASS <tool>` or `FAIL <tool>`; the last line of a failed
+   run names them:
+   `Quality checks failed: dotnet format --verify-no-changes`.
+4. A finding is written by the tool itself as `path(line,column): CODE message`.
+
+## Typical failures and fixes
+
+| Tool | What the log shows | Fix |
+| --- | --- | --- |
+| `dotnet format --verify-no-changes` | `Program.cs(12,5): error WHITESPACE: Fix whitespace formatting` | Run `dotnet format <project>` and commit |
+| `dotnet build -warnaserror` | `CS8602: Dereference of a possibly null reference` | Fix the code (null check), do not suppress the warning without a reason |
+| `dotnet build -warnaserror` | `IDE0005: Using directive is unnecessary` | Remove the directive |
+
+To reproduce a run on your machine, use the same SDK:
+
+```bash
+dotnet --version        # 10.0.401
+dotnet format src/App.sln --verify-no-changes
+dotnet build src/App.sln -warnaserror
+```
+
+## When the workflow itself fails
+
+| Message | Cause | Fix |
+| --- | --- | --- |
+| `[configuration] dotnet-sdk-version '<x>' is not an exact stable version 10 or later` | A channel, a range, a preview or a version below 10 | Give an exact version such as `10.0.401` |
+| `[configuration] working-directory must be a relative path inside the repository.` | An absolute path or `..` | Use a path inside the repository |
+| `[configuration] C# files exist but no solution or project was found` | The repository has `*.cs` files and no `*.sln`, `*.slnx` or `*.csproj` in the working directory | Add one, or set `working-directory` |
+| `[configuration] Several solutions or projects were found` | More than one candidate | Set the input `project` |
+| The install step fails | `dot.net` or the runner network is unreachable | Re-run the job; see [troubleshooting.md](troubleshooting.md) |