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>

packages/halkyon_llm/CHANGELOG.md

@@ -3,4 +3,4 @@
 * One interface (`LlmClient`) over Anthropic's Messages API, OpenAI's chat completions
   (with each known host's fields, and the local servers') and OpenAI's Responses API,
   streamed over plain HTTP; failures by kind; `FakeLlmClient` for tests; the recording
-  tool.
+  tool; `LlmEndpoint.onThisMachine`.