[WinUI OSS] Document public and scheduled build dependency differences

@harshit7962 is already working on this.

Since Jul 24, 2026.

## Summary

Document dependency differences between public development builds and scheduled/internal build workflows in public-safe language.

## Context

The ADO title mentions specific feed divergence scenarios for internal review. The public GitHub issue should avoid private feed names while still explaining the engineering problem: different build modes may see different package availability or timing, and developers need to know which workflow validates which dependency set.

## Work to do

- Describe build-mode dependency differences without naming private feeds.
- Identify risks caused by package version skew or deprecation timing.
- Document which validation path developers should use for public changes.
- Track follow-up work for package configuration or restore validation.
- Coordinate with dependency strategy and package restore tasks.

## Definition of done

- Dependency divergence risks are documented safely.
- Developers understand which build mode validates which package assumptions.
- Follow-up tasks exist for unresolved risks.
- Public text avoids private feed names and internal package timelines.