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>

testing/integration/src/test/kotlin/ClarificationLoopTest.kt

@@ -181,6 +181,39 @@ class ClarificationLoopTest {
         )
     }
 
+    @Test
+    fun `pendingClarificationFor resolves the live request server-side, then clears once answered`():
+        Unit = runBlocking {
+        // Backs the REST /clarify route (#42): a headless caller has no ClarificationRequestedEvent,
+        // so the server must resolve (stageId, requestId) from just the session id, and stop
+        // resolving it once answered.
+        val sessionId = SessionId("clarify-rest")
+        val orchestrator = orchestrator(listOf(withQuestions, clean))
+        val runJob = launch { orchestrator.run(sessionId, graph(), config) }
+
+        val pending = withTimeout(5_000) {
+            var p: ClarificationRequestedEvent? = null
+            while (p == null) {
+                p = orchestrator.pendingClarificationFor(sessionId)
+                if (p == null) yield()
+            }
+            p
+        }
+        assertEquals(analystStage, pending.stageId)
+        assertEquals("Which stack?", pending.questions.single().prompt)
+
+        orchestrator.submitClarification(
+            sessionId, pending.stageId, pending.requestId,
+            listOf(ClarificationAnswer(questionId = "stack", value = "React")),
+        )
+        runJob.join()
+
+        assertEquals(
+            null, orchestrator.pendingClarificationFor(sessionId),
+            "Answered clarification must no longer resolve as pending",
+        )
+    }
+
     @Test
     fun `a stage that emits questions parks once and is never re-run`(): Unit = runBlocking {
         val sessionId = SessionId("clarify-no-rerun")