Fix dependabot PR: move Aspire.AppHost.Sdk version to global.json

[Copilot]

Summary

Dependabot PR #3 failed CI with:

Newer version of the Aspire.Hosting.AppHost package is required. Ensure you are referencing at least version '13.3.3'.

Root cause: GillyTracker.AppHost.csproj hardcodes the Aspire.AppHost.Sdk version inline (<Project Sdk="Aspire.AppHost.Sdk/13.3.5">). Dependabot cannot update this format — it only updates Directory.Packages.props, .config/dotnet-tools.json, and global.json. So when dependabot bumped packages to 13.3.3, the SDK stayed behind at 13.2.0, causing the DCP runtime version check to fail.

Changes

• global.json: Added msbuild-sdks section with "Aspire.AppHost.Sdk": "13.3.5". Dependabot tracks this section for nuget updates, so future bumps will keep the SDK in sync with packages.
• GillyTracker.AppHost/GillyTracker.AppHost.csproj: Removed inline version from SDK reference (Aspire.AppHost.Sdk/13.3.5 → Aspire.AppHost.Sdk). Version is now resolved from global.json.

Testing

• Core tests pass
• AppHost builds successfully with SDK resolved from global.json
• Future dependabot PRs will update the SDK version alongside packages, preventing the mismatch that caused PR Bump the nuget group with 8 updates #3 to fail

Closes #3