Backlog: Document PS5 local test limitation and required validation path

[lipkau]

Problem\nContributors on macOS/Linux cannot run Windows PowerShell 5.1 test lanes locally, but this is easy to miss during development.\n\n## Proposal\nAdd explicit guidance stating that PSv5 validation must run in CI and define the expected local + CI validation flow.\n\n## Acceptance criteria\n- Contributor docs include a clear PSv5 limitation note.\n- The note includes the required CI check to watch before merge.\n- The guidance is placed near existing testing instructions.