fix: complete all P2 audit findings — dead code, NPE guard, hygiene

P2-1: Guard SessionId NPE with safe-call in SessionsReducer
P2-2: Remove 16 dead event classes + reducer branches; replace with live orchestration events in tests
P2-3: Standardize ChatInput divergences — guard CancelSession, single-arg ProtocolError
P2-4: Remove dead TuiToolRecord.diff and ToolDisplayStatus.REQUESTED
P2-5: Remove dead streamLive from SessionEventBridge
P2-6: Extract SessionsReducerContext data class for 6+ param method
P2-7: Replace StageId("none") sentinel with TypeId.NONE
P2-9: Add TypeId.random() factory on all type-alias IDs
P2-10: Add KDoc on schemaVersion documenting reserved-for-migration
P2-8: Verified RouterReducer string-template already correct (TypeId value class toString)

infrastructure/persistence/src/main/kotlin/com/correx/infrastructure/persistence/artifact/LiveArtifactRepository.kt

@@ -4,11 +4,7 @@ import com.correx.core.artifacts.ArtifactProjector
 import com.correx.core.artifacts.ArtifactReducer
 import com.correx.core.artifacts.ArtifactState
 import com.correx.core.artifacts.repository.ArtifactRepository
-import com.correx.core.events.events.ArtifactArchivedEvent
 import com.correx.core.events.events.ArtifactCreatedEvent
-import com.correx.core.events.events.ArtifactRejectedEvent
-import com.correx.core.events.events.ArtifactRelationshipAddedEvent
-import com.correx.core.events.events.ArtifactSupersededEvent
 import com.correx.core.events.events.ArtifactValidatedEvent
 import com.correx.core.events.events.ArtifactValidatingEvent
 import com.correx.core.events.events.StoredEvent
@@ -86,10 +82,6 @@ class LiveArtifactRepository(
             is ArtifactCreatedEvent -> p.artifactId
             is ArtifactValidatingEvent -> p.artifactId
             is ArtifactValidatedEvent -> p.artifactId
-            is ArtifactRejectedEvent -> p.artifactId
-            is ArtifactSupersededEvent -> p.artifactId
-            is ArtifactArchivedEvent -> p.artifactId
-            is ArtifactRelationshipAddedEvent -> p.sourceId
             else -> null
         }
 }