ADR 0203: the account's environment is one module's (seat node-environment);
every module contributes variables and PATH entries, rendered by the
controller as a POSIX file and as environment.d.
ADR 0204: shell code is contributed to the login shell in named slots, and
login-shell becomes the mesh's node-login-shell.
ADR 0205: software the distribution does not package ships as a pinned
archive of the module.
Issue 225: undeclaring a user stops a node applying; the shell is never
given back or checked.
To-be 41 carries the work packages; to-be 38 WP5 points to it.
