/handoff mints a fresh session via newSession(), producing a new
artifactsDir and an empty local/ root. The handoff document routinely
references plans and scratch files under '/data/workspaces/can1357__oh-my-pi__8261/.omp-session/2026-08-11T16-39-09-489Z_019ff1b1-31b1-7000-81f5-c540f4ebf43d/local/,' so every reference
became a dangling pointer in the new session. The plan approve-and-execute
path already copies artifacts across the boundary; handoff did not.
Extracted the plan-approve copy helper into a shared copyLocalArtifacts()
in local-protocol.ts and invoke it across the handoff session switch
(best-effort, since the switch is already committed).
Fixes#8261
roboomp
dbc199d2db