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)

core/artifacts/src/main/kotlin/com/correx/core/artifacts/ArtifactReducer.kt

@@ -1,11 +1,7 @@
 package com.correx.core.artifacts
 
 import com.correx.core.artifacts.model.ArtifactRelationship
-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
@@ -25,23 +21,6 @@ class DefaultArtifactReducer : ArtifactReducer {
                 transition(state, ArtifactLifecyclePhase.VALIDATING, ArtifactLifecyclePhase.CREATED)
             is ArtifactValidatedEvent ->
                 transition(state, ArtifactLifecyclePhase.VALIDATED, ArtifactLifecyclePhase.VALIDATING)
-            is ArtifactRejectedEvent ->
-                transition(state, ArtifactLifecyclePhase.REJECTED, ArtifactLifecyclePhase.VALIDATING)
-            is ArtifactSupersededEvent ->
-                transition(state, ArtifactLifecyclePhase.SUPERSEDED, ArtifactLifecyclePhase.VALIDATED)
-            is ArtifactArchivedEvent ->
-                transitionFromAny(
-                    state,
-                    ArtifactLifecyclePhase.ARCHIVED,
-                    ArtifactLifecyclePhase.VALIDATED,
-                    ArtifactLifecyclePhase.REJECTED,
-                )
-            is ArtifactRelationshipAddedEvent -> {
-                val rel = ArtifactRelationship(p.sourceId, p.targetId, p.relationshipType)
-                Result.success(
-                    state.copy(lineage = state.lineage.copy(relationships = state.lineage.relationships + rel))
-                )
-            }
             else -> Result.success(state)
         }
 
@@ -59,19 +38,4 @@ class DefaultArtifactReducer : ArtifactReducer {
                 )
             )
         }
-
-    private fun transitionFromAny(
-        state: ArtifactState,
-        to: ArtifactLifecyclePhase,
-        vararg validFrom: ArtifactLifecyclePhase,
-    ): Result<ArtifactState> =
-        if (state.phase in validFrom) {
-            Result.success(state.copy(phase = to))
-        } else {
-            Result.failure(
-                IllegalStateException(
-                    "Invalid artifact transition: ${state.phase} → $to (expected one of ${validFrom.toList()})"
-                )
-            )
-        }
 }