docs: the assistant answering in Learn, as step 3 built it

Learn's AGENTS.md describes the field with the assistants answering in it
(ADR 0021, step 3): who answers, the scope, the conversation sheet and its
lookups, images, private studysets' warnings, kept conversations and the
tasks; and the testing trap of awaiting a finished subscription's cancel.
The shared packages' READMEs name providerTermsOf and onThisMachine.
Learn's README no longer says there is no end-to-end encryption, which
private studysets changed (ADR 0019).

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/README.md

@@ -16,9 +16,11 @@ instance are published, so anyone can see how an instance works and run their ow
   email address with a password and an authenticator, the password never leaving the
   device ([ADR 0020](../../docs/adr/0020-sign-in-with-simplelogin.md),
   [ADR 0017](../../docs/adr/0017-password-totp-and-keys.md)).
-- **No end-to-end encryption.** Content is readable by the operator of the instance, as on
-  Mastodon; whoever trusts no operator can run their own
-  ([ADR 0002](../../docs/adr/0002-no-end-to-end-encryption.md)).
+- **Private studysets are end-to-end encrypted.** A private studyset's content is sealed
+  on the device with the account's keys; what isn't private, and what is published, the
+  operator of the instance can read, as on Mastodon
+  ([ADR 0019](../../docs/adr/0019-private-studysets.md),
+  [ADR 0002](../../docs/adr/0002-no-end-to-end-encryption.md)).
 
 Targets: Linux, Windows, macOS, Android and iOS. There is no web build.