Add portable session handoff

[ariobarin]

Summary

• add Matt Pocock's explicit-only handoff workflow
• keep handoff files in the OS temp directory instead of polluting the repository
• reference authoritative artifacts rather than copying them
• include suggested skills for the fresh agent

Evidence

• native Sol prepared HANDOFF.md inside the repository
• with handoff, the same scenario targeted the Windows temp directory, referenced docs/spec.md and commit �b41ab8 instead of duplicating the CSV contract, scoped only the remaining work, and suggested est-for-risk`n- git diff --check, pwsh tests, and Windows PowerShell 5.1 tests pass