[WinUI OSS] Track build configuration from GitHub source

## Summary

Track build configuration work needed for scheduled and release-oriented workflows that consume source from the GitHub repository layout.

## Context

Public repo files such as `build/WinUI-GitHub-PR.yml`, `Build.cmd`, `eng/muxfinalrelease.props`, `docs/publishing/build-pipelines.md`, and `docs/publishing/release-channels.md` provide public anchors for this work. Engineers should review which build configuration belongs in GitHub, which configuration is selected by parameters, and how scheduled/release-oriented workflows consume the source tree.

## Work to do

- Review public build configuration entry points for GitHub source assumptions.
- Identify configuration needed for nightly and release-oriented build modes.
- Make build mode selection explicit where possible.
- Track gaps that depend on package restore, validation, or packaging tasks.
- Keep private pipeline names, queue names, and release infrastructure details out of public issue text.

## Definition of done

- GitHub-hosted configuration needed by scheduled/release-oriented workflows is identified.
- Build mode selection is understandable to maintainers.
- Follow-up tasks exist for missing configuration.
- Public content remains safe and actionable.