docs(client): add «Кэш списка бесед» section + ios targets to :journal-inmemory

- :journal-inmemory — added iosX64/iosArm64/iosSimulatorArm64 to the
  target set so :storage-inmemory (which now depends on it) can build
  for iOS. Pure `MutableMap`+`Mutex` impl, no I/O, fully portable.

- client/README.md — new «Кэш списка бесед» section:
  - shows `agent.conversationStore` as the read-only entry point;
  - demonstrates the «remote.listFlow → local.upsert + outbox.agentEvents
    → local apply» pattern (Created/Renamed/Touched/Deleted);
  - notes that the cache is built into `AgentikAgent` by default;
  - mentions `wrapWithLocalConversationCache` for custom stores (SQLite/JSON);
  - points to `agentikHttpClient(...).raw` as the escape-hatch for clients
    that want direct HTTP.

Also adds `HttpConversationStore.kt` to the index (the file existed on
disk but wasn't `git add`ed in the previous commit).

journal-inmemory/build.gradle.kts

@@ -15,6 +15,9 @@ kotlin {
     jvm()
     macosX64()
     macosArm64()
+    iosX64()
+    iosArm64()
+    iosSimulatorArm64()
     linuxX64()
     linuxArm64()
     mingwX64()