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_server/lib/src/auth/learn_base.dart

@@ -84,13 +84,21 @@ const List<PrivacyPagePoint> _privacyPagePoints = [
         'Your conversations with assistants are kept to sync them between '
         'your devices, encrypted on them with your key: the instance sees how '
         'many there are, how long and when they were written, never what was '
-        'said. A device that can’t encrypt keeps none.',
+        'said. A device that can’t encrypt keeps none. An assistant gets what '
+        'you choose to send with a question and, where you let it look '
+        'further, what it looks up there: never your progress, and a private '
+        'studyset only once you agreed to send it to that assistant.',
     german:
-        'Deine Gespräche mit Assistenten werden aufbewahrt, um sie zwischen '
+        'Deine Unterhaltungen mit Assistenten werden aufbewahrt, um sie '
+        'zwischen '
         'deinen Geräten abzugleichen, auf ihnen mit deinem Schlüssel '
         'verschlüsselt: Die Instanz sieht, wie viele es sind, wie lang und '
         'wann sie geschrieben wurden, nie, was gesagt wurde. Ein Gerät, das '
-        'nicht verschlüsseln kann, bewahrt keine auf.',
+        'nicht verschlüsseln kann, bewahrt keine auf. Ein Assistent bekommt, '
+        'was du mit einer Frage zu senden auswählst, und, wo du ihn weiter '
+        'suchen lässt, was er dort nachschlägt: nie deinen Fortschritt, und ein '
+        'privates Lernset erst, nachdem du zugestimmt hast, es diesem '
+        'Assistenten zu senden.',
   ),
   (
     english: