feat(learn): a private studyset goes to an assistant only once agreed

ADR 0021, D9 and D17. Before a question sends a private studyset's
content to a remote model, through the parts chosen or a wider reach
with private studysets switched on, a warning names the studysets, the
provider's host and its terms (where checked: Anthropic, OpenAI,
Google, OpenRouter), and is asked once per studyset and assistant: the
agreement is kept in the person's sealed settings (schema 13,
privateAgreements, folded only where both rows hold it), shown in the
studyset's privacy dialog with Take back, and marked on the folded
button while it holds. Follow-ups and Try again ask again once taken
back, as the scope keeps where the parts came from (partsFrom). No
warning for an own assistant on this machine (loopback or localhost,
halkyon_llm's LlmEndpoint.onThisMachine); the local network may be
shared.

Texts that step 3 made untrue: the consent sheet now says what an
assistant looks up goes too and that conversations are kept encrypted
for the person's devices; the privacy dialogs no longer mention
showing a studyset to Claude; the privacy page says what is looked up
and when a private studyset goes.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

apps/learn/halkyon_learn_flutter/test/feature/studyset/model/studyset_tree_test.dart

@@ -9,12 +9,14 @@ Studyset studyset(
   String? parent,
   String position = 'a0',
   DateTime? archivedAt,
+  bool private = false,
 }) => Studyset(
   id: id,
   content: StudysetContent.titled(id),
   parentId: parent,
   position: position,
   archivedAt: archivedAt,
+  isPrivate: private,
 );
 
 Iterable<String> ids(List<Studyset> studysets) =>
@@ -128,6 +130,26 @@ void main() {
     });
   });
 
+  test('a private studyset is private by its tree, or by itself where it '
+      'is a topic in a readable one (ADR 0019)', () {
+    final all = [
+      studyset('japanese', private: true),
+      studyset('kana', parent: 'japanese', private: true),
+      studyset('german'),
+      studyset('verbs', parent: 'german', private: true),
+      studyset('strong', parent: 'verbs', private: true),
+      studyset('a', parent: 'b', private: true),
+      studyset('b', parent: 'a', private: true),
+    ];
+
+    expect(privateTopOf('kana', all), 'japanese');
+    expect(privateTopOf('japanese', all), 'japanese');
+    expect(privateTopOf('strong', all), 'verbs');
+    expect(privateTopOf('german', all), isNull);
+    expect(privateTopOf('missing', all), isNull);
+    expect(privateTopOf('b', all), anyOf('a', 'b'), reason: 'a cycle ends');
+  });
+
   group('what a session on a studyset studies', () {
     test('is its subtree in tree order, without archived studysets and '
         'what is inside them', () {