Convert .sln to .slnx format

[Copilot]

Proposed Changes

Migrates all solution files from legacy .sln to XML-based .slnx format using dotnet sln migrate. Requires .NET SDK 9.0.306+ for tooling support.

Changes:

• Updated global.json SDK version from 8.0.100 to 9.0.306
• Migrated 3 solution files: root MiniScaffold.sln and template files in Content/{Library,Console}/
• Updated build scripts (build/build.fs and template build files) to reference .slnx
• Updated documentation to reflect new format
• Removed legacy .sln files

Result: 154 fewer lines across solution files (-73%), cleaner diffs, fewer merge conflicts.

Types of changes

• New feature (non-breaking change which adds functionality)

Checklist

• Build and tests pass locally
• I have added tests that prove my fix is effective or that my feature works (if appropriate)
• I have added necessary documentation (if appropriate)

Further comments

Integration tests confirm both library and console templates generate correctly with .slnx files (28/28 tests passed). The XML format is significantly more readable than the legacy text-based format and is the recommended approach per Microsoft's .NET 9 announcement.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Convert sln to slnx</issue_title>
<issue_description>Describe the bug

Based on https://devblogs.microsoft.com/dotnet/introducing-slnx-support-dotnet-cli/ convert all sln files to slnx files. Use dotnet sln migrate
</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Convert sln to slnx #313

---

💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.