finish SwiftData migration with @Query views

- Drive chat list, timeline and forward picker from SwiftData @Query
  instead of AppModel's in-memory arrays; inject the per-account
  ModelContext from RootView with an in-memory fallback.
- Physically delete locally deleted messages (context.delete) and
  purge rows marked deleted by older builds on store open, so @Query
  views never resurrect them.
- Delete the legacy JSON snapshot only after a successful import save
  and restore the completeUntilFirstUserAuthentication data protection
  attribute on the store file.
- Mirror the old ChatArchive tolerant decoding in LegacyArchiveImporter
  so snapshots from older schema versions (missing reactions,
  isGroupMessage, roster fields) still import.
- Cover the new behaviour in ArchiveStoreTests and guard it with new
  verify.sh invariants; update AGENTS.md, ARCHITECTURE.md and
  SECURITY.md.

AGENTS.md

@@ -8,11 +8,17 @@ and the Martin / MartinOMEMO libraries. The Xcode project is generated from
 
 - `Sources/App/` — iOS/macOS app entry point (`LumaApp.swift`).
 - `Sources/Shared/` — shared code:
-  - `Models/` — value types and pure policy enums (`ChatMessage`,
-    `ArchiveSyncPagination`, `ChatTypingPolicy`, …).
-  - `UI/` — SwiftUI views and components.
+  - `Models/` — value types and pure policy enums (`ArchiveSyncPagination`,
+    `ChatTypingPolicy`, …) plus the SwiftData `@Model` classes
+    (`ChatMessage`, `Conversation`).
+  - `UI/` — SwiftUI views; chat list, timeline and forward picker read
+    SwiftData through `@Query` (`MainChatView`, `ChatView`,
+    `ForwardMessageView`).
   - `XMPP/` — `XMPPService` (MAM/OMEMO/MUC), `LumaCallEngine`, OMEMO store.
-  - `Persistence/` — `ChatArchive` (JSON snapshot), preferences.
+  - `Persistence/` — `ArchiveStore` (per-account SwiftData container),
+    `ArchiveMetadataRecord` (durable MAM checkpoint metadata),
+    `LegacyArchiveImporter` (one-time legacy JSON snapshot migration),
+    preferences.
   - `Services/`, `Security/` — media, notifications, credentials.
 - `Sources/Watch/` — single-target watchOS app.
 - `Tests/` — XCTest unit tests.