fix(node): the handshake ack dropped one app's directories

The ack was assembled from its own tuple of application names, a copy of the
daemon's `APP_DIR_KEYS`, and the two had drifted: the copy was missing
`helloworld`. So the reference application — the one that exists to prove a new
application needs no special-casing — was the single application whose
configured folders never reached a client, which made the plugin claim false
exactly where it is demonstrated.

Fixed by removing the copy rather than syncing it. The ack now emits whatever
`<app>_directories` the group context carries, and `_app_directories_ctx` is the
only thing that puts one there, so the two cannot disagree again. The transport
names an application in one place, `ALLOWED_APPS`, which is enforcement rather
than a directory list.

The client had the same fault one layer up: `group-page.js` read three names by
hand from the ack while the live-update path beside it was already generic. It
derives the map from the ack's own keys now, so the fix reaches the settings
pane instead of stopping at the wire.

A first attempt moved the list to `roster.py`, where directory *storage* lives,
and `test_helloworld_proves_the_plugin_claim.py` refused it: the roster, the ops,
the config and the root set must name no application at all. That test is the
architecture's own guard and it was right — the list belongs on the daemon, which
is what wires a group's context, and everything downstream is derived from it.

Two new tests, both verified to fail against the previous shape: the ack carries
an application the node names nowhere else, and the ack keeps no list of its own.
`test_the_lists_are_read_under_one_name_each` now asserts the shell names no
application rather than that it names exactly three.

Two stale comments went with it — the ack's, which described scalars removed in
07ff8b4, and the client's, which said those scalars still rode the wire for
MNP 1.0 peers that can no longer connect.

Full suite: 2258 passed, 4 skipped.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01YVoHVCcfBqud6ZjG4db3y7

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