Enable auto-merge for PRs targeting main/master and add Windows desktop workload to CI

[watermann420]

Motivation

• Ensure CI builds for WPF/WinForms projects by installing the Windows Desktop workload before building on Windows runners.
• Automatically enable GitHub auto-merge for eligible pull requests to reduce manual merges for changes targeting the main/master branches.

Description

• Add dotnet workload install windowsdesktop to the Windows job in .github/workflows/ci.yml so WPF/WinForms workloads are available during the CI build.
• Add a new workflow .github/workflows/auto-merge.yml that listens to PR events, completed check_suite, and workflow_run for CI and enables squash auto-merge via the GitHub GraphQL API for non-draft PRs.
• The auto-merge workflow only enables auto-merge for PRs targeting main or master and skips PRs from forks or draft PRs.
• Grant the workflow contents: write and pull-requests: write permissions to allow enabling auto-merge.

Testing

• No automated tests were run locally because these are GitHub Actions workflow changes and validation will occur when the workflows run in GitHub Actions.

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Codex Task