ExpandSessionDir kept a `{home}/<dotdir>` fallback for the case where
kandevHomeDir was unresolved. In production, ResolvedHomeDir() always
returns a non-empty path, so the fallback only ever fired in tests
that hadn't been updated to pass an instance ID. Returning the empty
string in that case makes container.go skip the bind mount — the
correct behaviour, since without a kandev home there's no session dir
to mount.
Every ACP agent's session dir was bind-mounted from ~/.<agent> on the
host into /root/.<agent> in the container. That exposed host state DBs,
session caches, and lock files to the container — codex's rollout DB
caches absolute /Users/... paths, which don't resolve inside Linux and
the agent silently loses prior thread context on resume.
Replace the host bind mount with a kandev-managed per-container session
dir under <kandev-home>/agent-sessions/<instance_id>/. Before the
container starts, selectively copy the auth-relevant files (the same
files Sprites already copies via RemoteAuth.SourceFiles) into that
dir; everything else — state DBs, sessions/, lock files — stays on the
host and the agent rebuilds them inside the container with
container-local Linux paths.
The change is agent-agnostic: any agent that has SessionDirTemplate
+ SessionDirTarget gets the new behavior automatically. Cleanup runs
on destructive stops only (task/session deleted/archived), mirroring
the Sprites preserve-on-stop rule so plain stops let resume re-attach
to the same agent state.
José Almeida