Add GET /conversations/{id}/count endpoint for total/filtered message counts, update JournalStore API, and implement client/server support with tests.

client/src/commonMain/kotlin/pw/binom/agentik/client/HttpJournalStore.kt

@@ -5,6 +5,7 @@ import io.ktor.client.call.body
 import io.ktor.client.request.get
 import io.ktor.client.request.parameter
 import io.ktor.http.HttpStatusCode
+import kotlinx.serialization.Serializable
 import pw.binom.agentik.journal.JournalStore
 import pw.binom.agentik.journal.MessageRecord
 import kotlin.time.Instant
@@ -13,8 +14,11 @@ import kotlin.time.Instant
  * HTTP-реализация [JournalStore] (append-only audit log сообщений диалога),
  * ходящая в `:server`-фасад.
  *
- * **Endpoint**: `GET {baseUrl}/journal/conversations/{id}/messages?after=&offset=&limit=`
- * (см. [pw.binom.agentik.server.journalRoutes]).
+ * **Endpoints** (см. [pw.binom.agentik.server.journalRoutes]):
+ *  - `GET {baseUrl}/journal/conversations/{id}/messages?after=&offset=&limit=`
+ *    → [list]
+ *  - `GET {baseUrl}/journal/conversations/{id}/count` → [count] (total)
+ *  - `GET {baseUrl}/journal/conversations/{id}/count?after=` → [count] (after cursor)
  *
  * Возвращает raw [MessageRecord] (все типы: UserMessage / AssistantMessage /
  * ToolCall / ToolResult / Error). В отличие от `GET /conversations/{id}/messages`
@@ -54,7 +58,28 @@ internal class HttpJournalStore(
         return response.body<List<MessageRecord>>()
     }
 
+    override suspend fun count(conversationId: String): Long {
+        val response = httpClient.get("$agentUrl/journal/conversations/$conversationId/count")
+        check(response.status == HttpStatusCode.OK) {
+            "journal.count: server returned ${response.status}"
+        }
+        return response.body<CountResponse>().count
+    }
+
+    override suspend fun count(conversationId: String, after: Instant): Long {
+        val response = httpClient.get("$agentUrl/journal/conversations/$conversationId/count") {
+            parameter("after", after.toString())
+        }
+        check(response.status == HttpStatusCode.OK) {
+            "journal.count(after): server returned ${response.status}"
+        }
+        return response.body<CountResponse>().count
+    }
+
     override fun close() {
         // HttpClient закрывает владелец (AgentClient / AgentikAgent).
     }
 }
+
+@Serializable
+private data class CountResponse(val count: Long)