feat(kernel): brief echo-back gate (C-A1)

- BriefEcho model + BriefEchoMismatchEvent (registered + serialization test)
- BriefEchoExtractor: parses the planner's brief_echo block and the original
  brief's referenced-files/symbols/acceptance-criteria dimensions
- BriefEchoComparator: conservative diff — a dropped acceptance criterion or an
  invented file path halts; dropping a file/symbol alone does not
- checkBriefEcho gate chained into the post-stage flow after groundBriefReferences;
  opt-in via stageConfig.metadata["briefEcho"], fails the stage (retryable) on
  divergence with re-grounding feedback
- prompt/workflow activation (planner.md emitting brief_echo; role_pipeline
  metadata) left as live-QA follow-up

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

core/events/src/main/kotlin/com/correx/core/events/events/BriefEchoEvents.kt

@@ -0,0 +1,38 @@
+package com.correx.core.events.events
+
+import com.correx.core.events.types.SessionId
+import com.correx.core.events.types.StageId
+import kotlinx.serialization.SerialName
+import kotlinx.serialization.Serializable
+
+/**
+ * A planner's structured restatement of the brief it was handed: the files it must touch, the
+ * symbols it must reach, and the acceptance criteria it must satisfy. The harness diffs this echo
+ * against the original brief before any plan is generated, so a divergence halts at the cheapest
+ * point in the pipeline. Also reused as the diff input shape for both sides of the comparison.
+ */
+@Serializable
+data class BriefEcho(
+    val referencedFiles: Set<String> = emptySet(),
+    val symbols: Set<String> = emptySet(),
+    val acceptanceCriteria: Set<String> = emptySet(),
+)
+
+/**
+ * A stage echoed the brief but the echo diverged from the original, so the plan is rejected
+ * before generation ("halt before plan generation"). Recorded so replay explains the halt.
+ *
+ * - [droppedFiles] / [droppedSymbols] / [droppedAcceptanceCriteria]: present in the original brief,
+ *   missing from the echo — the planner failed to carry them forward.
+ * - [inventedFiles]: file paths in the echo that are not in the original brief — hallucinated paths.
+ */
+@Serializable
+@SerialName("BriefEchoMismatch")
+data class BriefEchoMismatchEvent(
+    val sessionId: SessionId,
+    val stageId: StageId,
+    val droppedFiles: Set<String>,
+    val droppedSymbols: Set<String>,
+    val droppedAcceptanceCriteria: Set<String>,
+    val inventedFiles: Set<String>,
+) : EventPayload

core/events/src/main/kotlin/com/correx/core/events/serialization/Serialization.kt

@@ -8,6 +8,7 @@ import com.correx.core.events.events.ArtifactContentStoredEvent
 import com.correx.core.events.events.ArtifactCreatedEvent
 import com.correx.core.events.events.ArtifactValidatedEvent
 import com.correx.core.events.events.ArtifactValidatingEvent
+import com.correx.core.events.events.BriefEchoMismatchEvent
 import com.correx.core.events.events.BriefGroundingCheckedEvent
 import com.correx.core.events.events.ChatSessionStartedEvent
 import com.correx.core.events.events.ClarificationAnsweredEvent
@@ -109,6 +110,7 @@ val eventModule = SerializersModule {
         subclass(WorkspaceStateObservedEvent::class)
         subclass(RepoKnowledgeRetrievedEvent::class)
         subclass(BriefGroundingCheckedEvent::class)
+        subclass(BriefEchoMismatchEvent::class)
         subclass(RiskAssessedEvent::class)
         subclass(ChatSessionStartedEvent::class)
         subclass(ChatTurnEvent::class)

core/events/src/test/kotlin/com/correx/core/events/serialization/BriefEchoEventSerializationTest.kt

@@ -0,0 +1,28 @@
+package com.correx.core.events.serialization
+
+import com.correx.core.events.events.BriefEchoMismatchEvent
+import com.correx.core.events.events.EventPayload
+import com.correx.core.events.types.SessionId
+import com.correx.core.events.types.StageId
+import kotlin.test.Test
+import kotlin.test.assertEquals
+import kotlin.test.assertTrue
+
+class BriefEchoEventSerializationTest {
+
+    @Test
+    fun `BriefEchoMismatchEvent round-trips as polymorphic EventPayload`() {
+        val sample: EventPayload = BriefEchoMismatchEvent(
+            sessionId = SessionId("s"),
+            stageId = StageId("plan"),
+            droppedFiles = setOf("core/kernel/Foo.kt"),
+            droppedSymbols = setOf("Foo"),
+            droppedAcceptanceCriteria = setOf("must compile"),
+            inventedFiles = setOf("core/kernel/Bar.kt"),
+        )
+        val encoded = eventJson.encodeToString(EventPayload.serializer(), sample)
+        assertTrue(encoded.contains("\"type\":\"BriefEchoMismatch\""), "SerialName must be present: $encoded")
+        val decoded = eventJson.decodeFromString(EventPayload.serializer(), encoded)
+        assertEquals(sample, decoded)
+    }
+}