.NET: Improve skill name validation: reject consecutive hyphens and enforce directory name match

[SergeyMenshykh]

This PR improves skill validation in FileAgentSkillLoader with two changes:

1. Reject consecutive hyphens in skill names — The name regex now disallows names like "my--skill". The error message is updated accordingly.
2. Enforce skill name matches parent directory name — A new validation ensures the name field in SKILL.md frontmatter matches the parent directory name (e.g., a skill at skills/my-skill/SKILL.md must have name: my-skill). A mismatch logs an error and
   excludes the skill.

Closes: #4421

[Copilot]

Pull request overview

Updates the .NET skill loader’s validation to better align with the AgentSkills spec by tightening allowed skill-name formats and ensuring the frontmatter name matches the on-disk skill directory name.

Changes:

• Disallow consecutive hyphens in name via an updated validation regex and error message.
• Enforce name in SKILL.md frontmatter matches the parent directory name; mismatches are logged and excluded.
• Extend unit tests to cover consecutive-hyphen rejection and directory/name mismatch exclusion.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
dotnet/src/Microsoft.Agents.AI/Skills/FileAgentSkillLoader.cs	Tightens name regex and adds directory-name match validation + logging.
dotnet/tests/Microsoft.Agents.AI.UnitTests/AgentSkills/FileAgentSkillLoaderTests.cs	Adds/adjusts tests for consecutive hyphens, duplicate discovery, and directory/name mismatch.