fix: store actual rendered prompt artifact instead of contextpack.toString()

Extract message-rendering logic from LlamaCppInferenceProvider into a
provider-agnostic PromptRenderer in core:inference. SessionOrchestrator
now serializes the rendered ChatMessage list as JSON for the prompt
artifact, so the audit trail reflects what the LLM actually saw.

Resolves the TODO at SessionOrchestrator.kt:691.

core/kernel/src/main/kotlin/com/correx/core/kernel/orchestration/SessionOrchestrator.kt

@@ -61,6 +61,7 @@ import com.correx.core.inference.InferenceRepository
 import com.correx.core.inference.InferenceRequest
 import com.correx.core.inference.InferenceResponse
 import com.correx.core.inference.InferenceRouter
+import com.correx.core.inference.PromptRenderer
 import com.correx.core.inference.ResponseFormat
 import com.correx.core.inference.Tokenizer
 import com.correx.core.inference.ToolCallRequest
@@ -91,6 +92,7 @@ import kotlinx.coroutines.TimeoutCancellationException
 import kotlinx.coroutines.withContext
 import kotlinx.coroutines.withTimeout
 import kotlinx.datetime.Clock
+import kotlinx.serialization.encodeToString
 import kotlinx.serialization.json.Json
 import kotlinx.serialization.json.JsonObject
 import org.slf4j.LoggerFactory
@@ -688,8 +690,8 @@ abstract class SessionOrchestrator(
                 ),
         )
 
-        // TODO: capture rendered prompt; currently contextpack.toString()
-        val promptArtifactId = artifactStore.put(contextPack.toString().toByteArray())
+        val renderedMessages = PromptRenderer.render(contextPack)
+        val promptArtifactId = artifactStore.put(Json.encodeToString(renderedMessages).toByteArray())
         emit(sessionId, InferenceStartedEvent(requestId, sessionId, stageId, provider.id, promptArtifactId))
 
         if (isCancelled(sessionId)) return InferenceResult.Cancelled