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/context/src/main/kotlin/com/correx/core/context/DefaultContextReducer.kt

@@ -1,23 +1,8 @@
 package com.correx.core.context
 
 import com.correx.core.context.state.ContextState
-import com.correx.core.events.events.ContextBuildingFailedEvent
-import com.correx.core.events.events.ContextBuildingInterruptedEvent
-import com.correx.core.events.events.ContextBuildingStartedEvent
-import com.correx.core.events.events.ContextPackBuiltEvent
 import com.correx.core.events.events.StoredEvent
 
 class DefaultContextReducer : ContextReducer {
-    override fun reduce(state: ContextState, event: StoredEvent): ContextState =
-        when (val p = event.payload) {
-            is ContextBuildingStartedEvent -> state.copy(buildingInProgress = true, interrupted = false)
-            is ContextPackBuiltEvent -> state.copy(
-                buildingInProgress = false,
-                interrupted = false,
-                builtPackIds = state.builtPackIds + p.contextPackId,
-            )
-            is ContextBuildingFailedEvent -> state.copy(buildingInProgress = false, interrupted = false)
-            is ContextBuildingInterruptedEvent -> state.copy(buildingInProgress = false, interrupted = true)
-            else -> state
-        }
+    override fun reduce(state: ContextState, event: StoredEvent): ContextState = state
 }