feat(events): idea capture/discard events

IdeaCapturedEvent(ideaId, sessionId, text, timestampMs) + IdeaDiscardedEvent
(ideaId, sessionId) tombstone, registered in eventModule. The cross-session idea
board is rebuilt from these via eventStore.allEvents(); discard hides without
deleting (invariant #1).

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

@@ -0,0 +1,33 @@
+package com.correx.core.events.events
+
+import com.correx.core.events.types.SessionId
+import kotlinx.serialization.SerialName
+import kotlinx.serialization.Serializable
+
+/**
+ * An idea the operator surfaced while rubber-ducking with the router, captured to a cross-session
+ * scratchpad. Recorded as a fact (invariant #1) so the board is rebuilt from the log via
+ * `eventStore.allEvents()`. Non-authoritative: an idea informs the router's context but never drives
+ * the kernel — it is a note, not a decision.
+ */
+@Serializable
+@SerialName("IdeaCaptured")
+data class IdeaCapturedEvent(
+    val ideaId: String,
+    val sessionId: SessionId,
+    val text: String,
+    val timestampMs: Long,
+) : EventPayload
+
+/**
+ * The operator removed an idea from the board. The event log is append-only (invariant #1), so this
+ * is a tombstone — the original [IdeaCapturedEvent] stays in the log (and replays), but the idea-board
+ * projection filters out any [ideaId] that has a discard. [sessionId] is wherever the discard was
+ * issued (often a different session than the capture).
+ */
+@Serializable
+@SerialName("IdeaDiscarded")
+data class IdeaDiscardedEvent(
+    val ideaId: String,
+    val sessionId: SessionId,
+) : EventPayload

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

@@ -23,6 +23,8 @@ import com.correx.core.events.events.ExecutionPlanLockedEvent
 import com.correx.core.events.events.ExecutionPlanRejectedEvent
 import com.correx.core.events.events.HealthDegradedEvent
 import com.correx.core.events.events.HealthRestoredEvent
+import com.correx.core.events.events.IdeaCapturedEvent
+import com.correx.core.events.events.IdeaDiscardedEvent
 import com.correx.core.events.events.JournalCompactedEvent
 import com.correx.core.events.events.FileWrittenEvent
 import com.correx.core.events.events.L3MemoryRetrievedEvent
@@ -123,6 +125,8 @@ val eventModule = SerializersModule {
         subclass(ClarificationRequestedEvent::class)
         subclass(ClarificationAnsweredEvent::class)
         subclass(WorkflowProposedEvent::class)
+        subclass(IdeaCapturedEvent::class)
+        subclass(IdeaDiscardedEvent::class)
     }
 }