Add dotnet/skills marketplace and enable plugins

[Evangelink]

Add .github/copilot/settings.json to configure extra known marketplaces (dotnet/arcade-skills, dotnet/skills) and enable the following Copilot plugins:

• dotnet-dnceng (from dotnet-arcade-skills)
• dotnet, dotnet-test (from dotnet-skills)

[github-actions]

🚀 Dogfood this PR with:

⚠️ WARNING: Do not do this without first carefully reviewing the code of this PR to satisfy yourself it is safe.

curl -fsSL https://raw.githubusercontent.com/microsoft/aspire/main/eng/scripts/get-aspire-cli-pr.sh | bash -s -- 16356

Or

• Run remotely in PowerShell:

iex "& { $(irm https://raw.githubusercontent.com/microsoft/aspire/main/eng/scripts/get-aspire-cli-pr.ps1) } 16356"

[Copilot]

Pull request overview

Adds repository-level GitHub Copilot configuration to allow Copilot to discover additional skill marketplaces and to enable selected dotnet-related plugins.

Changes:

• Adds .github/copilot/settings.json configuring two extra known marketplaces (dotnet/arcade-skills, dotnet/skills)
• Enables three Copilot plugins: dotnet-dnceng, dotnet, and dotnet-test

Show a summary per file

File	Description
.github/copilot/settings.json	Registers additional Copilot skill marketplaces and enables specific plugins from them

Copilot's findings

• Files reviewed: 1/1 changed files
• Comments generated: 0