Reachability gains pid/workspace_dir, populated by enrich_reachability
whenever a backend's wait_ready() returns a pid: endpoint (currently
just ext.bwrap). build_reachability_report() now also returns a
local_exec_hint (nsenter into the pid's namespaces) alongside the
existing ssh_one_liner, so glas-harness can branch on which is
populated instead of assuming every sandbox is SSH-reachable.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>