Moved the cross-spawn-spawner module from packages/opencode to packages/core
to enable code sharing across the monorepo. This consolidates the process
spawning infrastructure into the core package so other packages can use
cross-platform child process spawning without duplicating the implementation.
Updated all import statements across the codebase to reference the new
location (@opencode-ai/core/effect/cross-spawn-spawner). Removed the
local copy from the opencode package along with its tests.
