The ACP backend resolved an unset cwd to the harness process's launch
directory, so a delegated child ran — and announced its ACP session —
in the server's cwd instead of the delegating session's workspace.
Resolve the working directory explicitly: the config cwd override
(made absolute and validated at load), else the parent session
header's cwd (validated at start), failing loud before spawning when
neither exists.
c432f85c07