add multithreaded task migration agent skill

[JanProvaznik]

Context

extracting docs and learnings from migration of copy for the agent to consume

[Copilot]

Pull request overview

This PR adds comprehensive documentation for migrating MSBuild tasks to support multithreaded execution. The documentation is structured as a skill guide that explains how to implement IMultiThreadableTask and use TaskEnvironment APIs to avoid global process state issues.

Changes:

• Adds a new skill document (.github/skills/multithreaded-task-migration/SKILL.md) that provides step-by-step migration guidance
• Documents the MSBuild multithreaded API including attribute usage, path resolution, environment variables, and process spawning
• Includes practical examples, test patterns, and a checklist for ensuring thread-safe task implementations

[AR-May]

Oh, forgot to add to my review - I noticed that copilot quite often does unnecessary switch from AbsolutePath to string. This leads to us moving between "string" and "AbsolutePath". It would be good if we tell it to try to stay in the "AbsolutePath" world as much as possible, so far as it does not hurt the edge cases.

[JanProvaznik]

merging now to have the baseline skill, if you run into something that copilot does wrong, ask it to add the lesson learned to the skill