refactor(content,core,app,editor,learn): Learn's content in pure Dart, for its agent

ADR 0044's D49 (amended in building it): what reads and seals Learn's
content moves out of the Flutter packages, so Learn's agent, which runs
without Flutter, shares it rather than repeating the privacy rules.

packages/halkyon_content (new, pure Dart) takes from halkyon_app the
envelope and its cipher, SealedFile, UnlockedAccountKeys, shared
spaces' gateway with SharedKeys and KeyPins, FileCache and
FileServerGateway, with their fakes and tests; halkyon_app exports it,
so no app changes. Its rich_text.dart takes RichContent from
halkyon_editor, which exports it. newAccountKeys makes an agent's key
pair. halkyon_core takes DelayedDeletions and gains SyncRequests, what a
synced gateway asks of the sync, which SyncService implements.

apps/learn/halkyon_learn_core (new, pure Dart) takes Learn's privacy,
sync, payloads and the models, codecs, gateways and repositories of
studysets, cards, notes, tags and feedback, with what they reach of
files, learning paths, citations and study options; the app imports them
from there. A subject's icon, colour and name stay in the app
(SubjectLook); what only the app shows (progress, exam coverage, the
card browser, deletions' waits) stays too. Nothing changes for people
using the apps.

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/AGENTS.md

@@ -4,7 +4,7 @@ What holds for every app and package of this repository (how work is done here,
 
 A spaced-repetition learning app: studysets of flashcards, reviewed on an FSRS schedule. Content is rich text (`appflowy_editor` documents) everywhere. Targets: Linux, Windows, macOS, Android, iOS. There is no web build.
 
-- **Packages** (`apps/learn/`): `halkyon_learn_server` (endpoints, models, migrations), `halkyon_learn_client` (generated, never edited), `halkyon_learn_flutter` (the app, ID `app.halkyon.learn`), `halkyon_learn_curator` (the dictionaries' converter, below), and `tool/` with the scripts that write the app's assets (maps, Anki fixtures; run them from `apps/learn`; its icon, Lucide's graduation cap since ADR 0031, is the root's `tool/app_icons.py`'s, `assets/icon/halkyon_learn_icon.png`, also atop the rail, on the launch screen and on halkyon_app's screens (`learnIcon`) since 2 Nov 2026, as Todo's and Train's show theirs, ADR 0031, D6). What every Halkyon server has comes from the Serverpod module `halkyon_base` (ADR 0014, decided 2026-10-19; `packages/halkyon_base_server` and `packages/halkyon_base_client`, which Learn's packages take by path): signing in, the account's ways in and its deletion, the pruning, and the instance's basics. The server sets it up for Learn before it starts (`HalkyonBase.configure(pod, learnBase(accounts: learnAccounts()))` in `server.dart`; `lib/src/auth/learn_base.dart`: Learn holds the instance's register of accounts (ADR 0016), `learnAccounts` reading the members from `INSTANCE_MEMBERS` and refusing to start with `ACCOUNT_REGISTER_URL`; the app it serves, `learn`, which its instances name in their basics and the app checks (never change it), the app's name in sign-in mails, this project's source, `minimumAppBuild`, Learn's own deletion as a hook), its endpoints are `client.modules.base` in the app, its tables (`email_identity`, `email_code_request`) are the module's, and tests that delete people or ask `instance.info()` set it up as the server does (`useLearnBase(sessionBuilder)`). What every Halkyon app's shell has comes from `halkyon_app` on the module's client (ADR 0014, `packages/halkyon_app`; with `halkyon_core`'s `SerialQueue` and `CoalescingRunner`): the instances and the first start, signing in and switching person, the gates (`InstanceGate`, `SessionGate`, `PersonScope`, `AppScope`), the sync service and where it stands, with texts of its own (`HalkyonAppLocalizations`, in `appLocalizationsDelegates`) that name the app from `learnFrame`. Change what they hold there, not here.
+- **Packages** (`apps/learn/`): `halkyon_learn_server` (endpoints, models, migrations), `halkyon_learn_client` (generated, never edited), `halkyon_learn_flutter` (the app, ID `app.halkyon.learn`), `halkyon_learn_core` (Learn's content in pure Dart, shared with its agent, Halkyon's ADR 0044, D49: privacy, sync, payloads, and the models, gateways and repositories of studysets, cards, notes, tags and feedback; the app imports them from `package:halkyon_learn_core/…`, and what only it shows stays in it), `halkyon_learn_curator` (the dictionaries' converter, below), and `tool/` with the scripts that write the app's assets (maps, Anki fixtures; run them from `apps/learn`; its icon, Lucide's graduation cap since ADR 0031, is the root's `tool/app_icons.py`'s, `assets/icon/halkyon_learn_icon.png`, also atop the rail, on the launch screen and on halkyon_app's screens (`learnIcon`) since 2 Nov 2026, as Todo's and Train's show theirs, ADR 0031, D6). What every Halkyon server has comes from the Serverpod module `halkyon_base` (ADR 0014, decided 2026-10-19; `packages/halkyon_base_server` and `packages/halkyon_base_client`, which Learn's packages take by path): signing in, the account's ways in and its deletion, the pruning, and the instance's basics. The server sets it up for Learn before it starts (`HalkyonBase.configure(pod, learnBase(accounts: learnAccounts()))` in `server.dart`; `lib/src/auth/learn_base.dart`: Learn holds the instance's register of accounts (ADR 0016), `learnAccounts` reading the members from `INSTANCE_MEMBERS` and refusing to start with `ACCOUNT_REGISTER_URL`; the app it serves, `learn`, which its instances name in their basics and the app checks (never change it), the app's name in sign-in mails, this project's source, `minimumAppBuild`, Learn's own deletion as a hook), its endpoints are `client.modules.base` in the app, its tables (`email_identity`, `email_code_request`) are the module's, and tests that delete people or ask `instance.info()` set it up as the server does (`useLearnBase(sessionBuilder)`). What every Halkyon app's shell has comes from `halkyon_app` on the module's client (ADR 0014, `packages/halkyon_app`; with `halkyon_core`'s `SerialQueue` and `CoalescingRunner`): the instances and the first start, signing in and switching person, the gates (`InstanceGate`, `SessionGate`, `PersonScope`, `AppScope`), the sync service and where it stands, with texts of its own (`HalkyonAppLocalizations`, in `appLocalizationsDelegates`) that name the app from `learnFrame`. Change what they hold there, not here.
 - **Instances**: anyone may run one. The app takes the server URL from `--dart-define=SERVER_URL=…`, so never hard-code a host.
 
 ## Working on Learn