.NET: Support Agent Skills

[SergeyMenshykh]

Summary

Adds FileAgentSkillsProvider, an AIContextProvider that discovers and exposes Agent Skills from filesystem directories following the Agent Skills specification progressive disclosure pattern: advertise → load → read resources.

Changes

• FileAgentSkillsProvider — discovers SKILL.md files from configured directories, advertises skills via system prompt injection, and provides load_skill / read_skill_resource tools for on-demand access.
• FileAgentSkillsProviderOptions — configuration for custom prompt templates.
• FileAgentSkillLoader / FileAgentSkill / SkillFrontmatter — internal helpers and model classes for skill discovery, frontmatter parsing, and secure resource reading (path traversal / symlink guards).
• Unit tests covering discovery, loading, resource reading, and security scenarios.
• Sample (Agent_Step01_BasicSkills) demonstrating usage with an expense-report skill.
• FileAgentSkillsProvider is marked as experimental (MAAI001) using DiagnosticIds.

Key Design Decisions

• Skills are read-only static content; no code execution.
• XML-escaping for prompt injection safety.
• Resource reads are guarded against path traversal and symlink escape.
• Only skills from trusted sources should be used.

Out of Scope

• Scripts execution, which will be added in one of the following PRs.

Closes: #3499

[Copilot]

Pull request overview

Adds a new .NET AIContextProvider that discovers Agent Skills from the filesystem and exposes them to agents via progressive disclosure (advertise → load → read resources), along with tests and a getting-started sample.

Changes:

• Introduces FileAgentSkillsProvider + supporting loader/types to discover SKILL.md skills, advertise them via injected instructions, and expose load_skill / read_skill_resource tools.
• Adds unit tests for discovery/parsing/resource-reading and updates projects to suppress the experimental diagnostic (MAAI001).
• Adds a new Getting Started “Agent Skills” sample and wires it into the samples index/solution.

Reviewed changes

Copilot reviewed 18 out of 18 changed files in this pull request and generated 9 comments.

Show a summary per file

File	Description
dotnet/tests/Microsoft.Agents.AI.UnitTests/Microsoft.Agents.AI.UnitTests.csproj	Suppresses experimental diagnostic in unit tests.
dotnet/tests/Microsoft.Agents.AI.UnitTests/AgentSkills/FileAgentSkillsProviderTests.cs	Tests provider prompt/tool injection behavior and customization.
dotnet/tests/Microsoft.Agents.AI.UnitTests/AgentSkills/FileAgentSkillLoaderTests.cs	Tests skill discovery/parsing/resource validation and read behavior.
dotnet/src/Microsoft.Agents.AI/Skills/SkillFrontmatter.cs	Adds parsed frontmatter type for skill metadata.
dotnet/src/Microsoft.Agents.AI/Skills/FileAgentSkillsProviderOptions.cs	Adds options for customizing the advertisement prompt template.
dotnet/src/Microsoft.Agents.AI/Skills/FileAgentSkillsProvider.cs	Implements the context provider and exposes the two skill tools.
dotnet/src/Microsoft.Agents.AI/Skills/FileAgentSkillLoader.cs	Implements discovery, frontmatter parsing, and resource validation/reading.
dotnet/src/Microsoft.Agents.AI/Skills/FileAgentSkill.cs	Adds internal representation of a loaded skill (body + resources).
dotnet/src/Microsoft.Agents.AI/Microsoft.Agents.AI.csproj	Enables diagnostic IDs/experimental attribute injection and suppresses MAAI001.
dotnet/samples/GettingStarted/README.md	Links new Agent Skills section from Getting Started index.
dotnet/samples/GettingStarted/AgentSkills/README.md	Adds index README for Agent Skills samples.
dotnet/samples/GettingStarted/AgentSkills/Agent_Step01_BasicSkills/skills/expense-report/references/POLICY_FAQ.md	Adds sample skill reference document.
dotnet/samples/GettingStarted/AgentSkills/Agent_Step01_BasicSkills/skills/expense-report/assets/expense-report-template.md	Adds sample skill asset template.
dotnet/samples/GettingStarted/AgentSkills/Agent_Step01_BasicSkills/skills/expense-report/SKILL.md	Adds sample skill with frontmatter + resource links.
dotnet/samples/GettingStarted/AgentSkills/Agent_Step01_BasicSkills/README.md	Documents the sample and how progressive disclosure works.
dotnet/samples/GettingStarted/AgentSkills/Agent_Step01_BasicSkills/Program.cs	Demonstrates usage of FileAgentSkillsProvider with an agent.
dotnet/samples/GettingStarted/AgentSkills/Agent_Step01_BasicSkills/Agent_Step01_BasicSkills.csproj	Adds sample project and copies skill content to output.
dotnet/agent-framework-dotnet.slnx	Adds the new sample to the solution structure.