[CI logging] Add fixtures and GitHub Actions dogfood workflow

### Summary

Add deterministic diagnostic fixtures and a GitHub Actions workflow for end-to-end CI logger validation.

### Background and Motivation

Unit tests can validate emitted workflow-command text, but the MVP must also prove that GitHub Actions recognizes the commands, displays annotations, and links them to the correct source locations.

The MSBuild repository is sufficient for this validation; a separate repository is not required.

### Proposed Feature

Add fixtures covering:

- compiler errors and warnings;
- an MSBuild-generated error;
- missing source locations;
- special characters and multiline messages;
- warning promotion;
- multiproject and parallel failures.

Add a locked-down GitHub Actions dogfood workflow that:

- starts as a manually triggered workflow;
- builds MSBuild first;
- runs the fixtures with bootstrap MSBuild;
- asserts expected success or failure and emitted workflow commands;
- runs on GitHub-hosted Linux and Windows;
- uses read-only permissions and no secrets.

Initial acceptance includes manual confirmation that annotations appear once and navigate to the expected repository file and location.

### Alternative Designs

A separate repository would add maintenance and version-coordination costs without improving MVP validation. Tests alone would not verify GitHub's native annotation rendering.

No se especifican archivos ni pruebas concretos; empieza por localizar el workflow de CI existente, las pruebas de registro de diagnóstico y el punto de entrada de bootstrap de MSBuild en el repositorio de MSBuild. Añade fixtures deterministas y un workflow de Linux y Windows de solo lectura, activado manualmente, que valide los resultados esperados y los comandos del workflow; después, confirma que las anotaciones aparecen una sola vez y enlazan con la ubicación de origen esperada.

Escrito por el modelo de indexación a partir del texto del issue.