refactor(message-log-api): split MessageStore into read-only and mutable interfaces

- Introduced `MutableMessageStore` for producers with an `append` operation, separate from read-only `MessageStore`.
- Updated all consumers and implementations to use the appropriate interface (`read-only` for observers, `mutable` for producers).
- Improves modularity and ensures compile-time guarantees against unintended write operations in the audit log.

storage-ksqlite/src/commonMain/kotlin/pw/binom/agentik/storage/ksqlite/KsqliteMessageStore.kt

@@ -2,7 +2,7 @@ package pw.binom.agentik.storage.ksqlite
 
 import kotlinx.serialization.json.Json
 import pw.binom.agentik.messageLog.MessageRecord
-import pw.binom.agentik.messageLog.MessageStore
+import pw.binom.agentik.messageLog.MutableMessageStore
 import pw.binom.agentik.messageLog.TokenStats
 import pw.binom.agentik.messageLog.decodeBodyPayload
 import pw.binom.agentik.messageLog.encodeBodyPayload
@@ -24,7 +24,7 @@ import kotlinx.coroutines.withContext
  */
 class KsqliteMessageStore(
     private val connection: SQLiteConnection,
-) : MessageStore {
+) : MutableMessageStore {
 
     private val mutex = Mutex()
     private val json = Json { ignoreUnknownKeys = true }

storage-ksqlite/src/commonMain/kotlin/pw/binom/agentik/storage/ksqlite/KsqliteStores.kt

@@ -1,7 +1,7 @@
 package pw.binom.agentik.storage.ksqlite
 
 import pw.binom.agentik.messageStore.ConversationStore
-import pw.binom.agentik.messageLog.MessageStore
+import pw.binom.agentik.messageLog.MutableMessageStore
 import pw.binom.agentik.messageStore.ReflectionStore
 import pw.binom.agentik.workingMemory.WorkingMemoryStore
 import pw.binom.db.ksqlite.SQLiteConnection
@@ -18,7 +18,7 @@ import pw.binom.db.ksqlite.SQLiteConnection
 class KsqliteStores private constructor(
     val connection: SQLiteConnection,
     val conversations: ConversationStore,
-    val messages: MessageStore,
+    val messages: MutableMessageStore,
     val workingMemory: WorkingMemoryStore,
     val reflections: ReflectionStore,
 ) : AutoCloseable {