Enhance messaging components, general code quality and sample applications

[tsutomi]

This pull request introduces two new architectural skills for .NET projects and adds supporting reference documentation for xUnit test organization. The most significant changes are the addition of comprehensive skill guides for structuring multi-library .NET frameworks and xUnit-based test solutions, as well as the inclusion of focused reference materials to aid agents in test organization and best practices.

New .NET Architectural Skills:

• Added custom-framework-arch skill, providing detailed guidance on structuring custom .NET frameworks composed of reusable libraries, including package boundaries, repository layout, API layering, build configuration, and documentation maintenance routines. (.agents/skills/custom-framework-arch/SKILL.md)
• Added xunit-test-arch skill, outlining conventions for organizing xUnit test projects, folder layout, shared MSBuild configuration, package selection per target framework, and coverage tooling setup for .NET class libraries. (.agents/skills/xunit-test-arch/SKILL.md)

xUnit Test Organization Reference Documentation:

• Introduced a references folder for the xunit-test-organization skill, containing an index and supplemental files for targeted guidance on topics such as xUnit usage, test runners, code coverage, data generation with Bogus, and MSBuild configuration. (.agents/skills/xunit-test-organization/references/README.md)
• Added a focused reference on using the Bogus library for randomized test data, including best practices for fakers, seeding, and integration with xUnit tests. (.agents/skills/xunit-test-organization/references/bogus.md)

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