Verify /handoff acceptance criteria on the Windows AVD

[vib795]

Spec and full acceptance criteria live in #1, closed as delivered. The items below need a real Windows AVD session with Copilot Enterprise and cannot be covered by CI.

• AC1 — /handoff invocable in a Copilot chat in any VS Code window, with no per-repo setup
• AC1 — the same from Claude Code, through the linked skill
• AC3 — cold-read test: a fresh session given only the handoff file recovers (a) the next action and (b) the rationale for decision /handoff — portable working-state capture for Copilot and Claude Code #1. This is the primary quality gate
• AC4 — a re-run leaves exactly <id>.md and <id>.prev.md, with no third version
• AC5 — a re-run with a drifted title updates the existing thread and names it in the output
• AC10 — an oversized thread splits into <id>.detail.md with no content lost
• AC11 — PowerShell paths hold with a space in the profile path
• Open risk from /handoff — portable working-state capture for Copilot and Claude Code #1 — whether Copilot agent mode's file-edit tool can write outside the workspace root, or must fall back to terminal execution

CI covers install, skill linking, and the store round trip on windows-latest, but it cannot cover Copilot invocation or the quality of a model-authored artifact. Those are the gaps.