feat(client): implement HttpJournalStore and integrate journal endpoints

- Added `HttpJournalStore` as an HTTP-backed implementation of `JournalStore` for read-only access to the audit log.
- Integrated `GET /journal/conversations/{id}/messages` endpoint to fetch conversation transcripts with full payloads.
- Updated `AgentClient` to expose `HttpJournalStore` as the `journal` property.
- Adjusted `HttpEventStore` to align with updated endpoint structure (`/outbox/events`).

client/build.gradle.kts

@@ -22,6 +22,7 @@ kotlin {
         commonMain.dependencies {
             api(project(":proto"))
             api(project(":event-store"))
+            api(project(":journal-api"))
 
             api(libs.ktor.client.core)
             implementation(libs.ktor.client.content.negotiation)