When a session is created with a cwd that doesn't exist, all tool
execution fails with a confusing 'fork/exec landrun: no such file or
directory' because exec.Command sets cmd.Dir to the non-existent path.
Validate that cwd exists and is a directory at session creation time,
returning a clear error instead of allowing a broken session.
Also includes the earlier USER/NAMESPACE env export fix (agent.go,
shell.go) for the ollie-9p namespace path issue.
a73899bc2f