[WinUI OSS] Track source and build/release model decisions

## Summary

Track decisions and research needed for the GitHub source and downstream build/release model.

## Context

This parent issue covers the operating model: how GitHub source flows into build validation, how scheduled builds differ from PR validation, how release-oriented validation is represented, and how WindowsAppSDK integration is coordinated. The public issue should explain decisions at a high level while keeping private service details out.

## Work to do

- Track child research tasks for comparable GitHub-to-build patterns.
- Confirm source synchronization and build/release precedent.
- Decide how validation, dependencies, and build modes are represented for GitHub development.
- Convert research outcomes into implementation tasks.
- Keep private project management, private repo URLs, and private pipeline details out of public content.

## Definition of done

- Key source/build/release decisions are captured.
- Child investigation tasks have actionable outcomes.
- Implementation follow-ups are created where needed.
- Maintainers understand the selected model at a high level.