[WinUI OSS] Determine dependency strategy for scheduled builds

## Summary

Determine the package dependency strategy for scheduled WinUI builds that consume GitHub source.

## Context

Public PR builds and scheduled builds can have different dependency needs. This task should clarify the strategy without naming private feeds or private package sources. It should coordinate with `NuGet.config` readiness, package restore validation, and WindowsAppSDK integration work.

## Work to do

- Identify dependency expectations for public PR builds versus scheduled builds.
- Decide which behavior should be documented for maintainers.
- Track build configuration changes needed to select the correct restore behavior.
- Coordinate with #11275, #11290, and #11298.
- Avoid private feed names, access instructions, and unpublished package details.

## Definition of done

- Scheduled build dependency strategy is clear.
- Differences from public PR builds are documented safely.
- Follow-up build/package tasks are linked.
- Engineers can start implementation without private context in the issue body.