feat(server): REST /clarify route for headless clarification answers (#42)

Discovery-stage clarifications could only be answered over WebSocket
(ClientMessage.ClarificationResponse), so the curl-based headless QA
driver parked forever at discovery.

Add POST /sessions/{id}/clarify mirroring approveStageRoute. The server
resolves the live (stageId, requestId) from the session id via
SessionOrchestrator.pendingClarificationFor() — the newest still-live,
unanswered ClarificationRequestedEvent from the event log — so a curl
caller need not know the requestId. Empty answers = free-text skip.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

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

@@ -9,6 +9,8 @@ import com.correx.core.artifactstore.ArtifactStore
 import com.correx.core.context.builder.ContextPackBuilder
 import com.correx.core.context.model.ContextPack
 import com.correx.core.events.events.ClarificationAnswer
+import com.correx.core.events.events.ClarificationAnsweredEvent
+import com.correx.core.events.events.ClarificationRequestedEvent
 import com.correx.core.events.events.InferenceCompletedEvent
 import com.correx.core.events.events.InferenceFailedEvent
 import com.correx.core.events.events.InferenceStartedEvent
@@ -260,6 +262,22 @@ abstract class SessionOrchestrator(
     fun liveClarificationRequestIds(): Set<String> =
         pendingClarifications.keys.mapTo(mutableSetOf()) { it.value }
 
+    /**
+     * The clarification a headless caller should answer for this session: the newest still-live,
+     * unanswered request. Null if the session is not parked on a clarification. Lets a REST answer
+     * route resolve the (stageId, requestId) server-side so a script need only supply answer values —
+     * the WS path knows them because it received the [ClarificationRequestedEvent]; a curl caller does not.
+     */
+    suspend fun pendingClarificationFor(sessionId: SessionId): ClarificationRequestedEvent? {
+        val live = liveClarificationRequestIds()
+        if (live.isEmpty()) return null
+        val events = eventStore.read(sessionId)
+        val answered = events.mapNotNull { it.payload as? ClarificationAnsweredEvent }
+            .mapTo(mutableSetOf()) { it.requestId }
+        return events.mapNotNull { it.payload as? ClarificationRequestedEvent }
+            .lastOrNull { it.requestId.value in live && it.requestId !in answered }
+    }
+
     /** Public seam for out-of-band gates (e.g. freestyle plan review) that reuse the stage-approval flow. */
     suspend fun requestPlanApproval(sessionId: SessionId, preview: String): Boolean =
         requestStageApproval(sessionId, StageId("plan_review"), preview)