.NET: Add tweaks to .net agent skills

[westey-m]

Motivation and Context

Description

• Add tweaks to .net agent skills

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the Contribution Guidelines
• All unit tests pass, and I have added new tests where possible
• Is this a breaking change? If yes, add "[BREAKING]" prefix to the title of the PR.

[Copilot]

Pull request overview

This pull request updates the .NET agent framework documentation to improve clarity and provide more detailed guidance for AI coding agents working with the codebase. The changes reorganize documentation structure by moving build/test instructions and project structure details into dedicated skill files, and adding new sections on core types and design principles.

Changes:

• Refactored AGENTS.md to reference separate skill files for build/test and project structure details
• Added documentation of core types (AIAgent, AgentSession, ChatClientAgent, etc.) and design principles (DRY, Single Responsibility, etc.)
• Enhanced build/test skill documentation with more examples and tips for package restore, multi-framework testing, and Linux-specific guidance

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
dotnet/AGENTS.md	Refactored to reference skill files; added core types section and design principles; contains incorrect reference to FunctionTool
dotnet/.github/skills/project-structure/SKILL.md	Added visual directory tree structure and updated provider descriptions
dotnet/.github/skills/build-and-test/SKILL.md	Enhanced with additional build examples, package restore tips, and Linux testing guidance; contains a typo