[WinUI OSS] Coordinate dependency cleanup for downstream integration

## Summary

Coordinate dependency cleanup needed for downstream WindowsAppSDK integration, especially around legacy transport package usage.

## Context

The public repo includes WindowsAppSDK-related files such as `docs/winappsdk-overview.md`, `build/nuspecs/Microsoft.WindowsAppSDK.WinUI.nuspec`, and `pack.component.cmd`. This task should focus on what WinUI needs to clean up or coordinate so downstream integration can consume GitHub-sourced changes cleanly.

## Work to do

- Identify dependency cleanup areas that affect WindowsAppSDK integration.
- Determine whether changes belong in WinUI, downstream integration, or both.
- Coordinate ownership and sequencing with related dependency tasks.
- Track concrete implementation follow-ups separately.
- Avoid private package names, private feeds, and internal aggregator implementation details.

## Definition of done

- Downstream dependency cleanup responsibilities are clear.
- Coordination points are identified.
- Follow-up implementation tasks are linked.
- Public issue content remains safe and actionable.