session: reject creation with non-existent cwd

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.

fs/session/create.go

@@ -63,6 +63,11 @@ func Create(rs *rootState, args []string) (string, error) {
 	if cwd == "" {
 		return "", fmt.Errorf("cwd is required (e.g. new cwd=/path/to/project)")
 	}
+	if info, err := os.Stat(cwd); err != nil {
+		return "", fmt.Errorf("cwd %q does not exist", cwd)
+	} else if !info.IsDir() {
+		return "", fmt.Errorf("cwd %q is not a directory", cwd)
+	}
 
 	sessID := name
 	if sessID == "" {