refactor(mnp)!: one operation for an app's folders, not one per app

`video_root`, `audio_root` and `photo_roots` are gone — the messages, the
signed operations, the handlers, the `ops` wrappers, the three scalars on the
handshake ack, and the client's handlers for their acks. `app_directories`
does the same thing for every application, keyed by the app's own registry
name, and it is what the SPA has been sending.

The three were the same instruction three times, differing only in the key they
wrote and whether they carried a string or a list. That shape is what made
adding an application mean adding a message type, an op, a handler and a widget;
it also meant three validation paths, and the older ones validated nothing —
a typo was stored and then quietly matched no entry, an app showing an empty tab
with no way to tell "misconfigured" from "no files yet".

**What stays, and why.** `Roster.LEGACY_DIR_KEYS` still reads `video_root` and
friends out of `group_settings`: that is a key on an operator's disk, not on the
wire, and a node upgraded into this must find its own configuration. The Search
page still reads its own older cache keys, for the same reason — the cache
outlives a deploy. `CTX_ALIASES` keeps only `chat`, which is the one app whose
second name something still reads.

The two per-app policy test files go with the messages. What only they held —
the real challenge/response path from message to database, which no other test
exercises — is retargeted at `app_directories` in
`test_app_directories_signed.py`, and the handler's own refusals (unknown app,
malformed `directories`, nobody to authorize it) join `test_app_directories.py`.

Node and common suites 1368 passed.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AsoWC3GmhNdwVFomW3QjH3

1 files changed, 3 insertions, 2 deletions

diff --git a/packages/meshbay-node/tests/test_video_root_gates_enrichment.py b/packages/meshbay-node/tests/test_video_root_gates_enrichment.py
index b88ecaf..fd0e040 100644
--- a/packages/meshbay-node/tests/test_video_root_gates_enrichment.py
+++ b/packages/meshbay-node/tests/test_video_root_gates_enrichment.py
@@ -6,7 +6,7 @@ rate limit and the node's CPU on an operator's whole shared index before
 they have chosen which folder is actually their media library would be
 real, ongoing cost for files never meant to be in the Videos app at all.
 
-Setting or changing the root (ops.set_video_root) fires a one-off sweep
+Setting or changing the folder (ops.set_app_directory) fires a one-off sweep
 (_enrich_video_root_now) of whatever it already contains: the ordinary
 per-broadcast path only ever looks at files new since the last broadcast,
 so anything already sitting in a folder before it became the video_root
@@ -144,7 +144,8 @@ async def test_setting_the_video_root_sweeps_what_it_already_contains(tmp_path):
             "groups_ctx": {group_id: {"roots": one_root(shared)}},
             "enrich_app_dirs_fns": {"video": daemon._enrich_video_root_now},
         }
-        await ops.set_video_root(state, group_id, "shared/Movies")
+        await ops.set_app_directory(state, group_id, "video",
+                                    "shared/Movies")
         await asyncio.sleep(0.05)  # let the fire-and-forget sweep actually run
 
         entry = next(iter(indexer.index.entries))