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/sessions/src/main/kotlin/com/correx/core/sessions/DefaultSessionReducer.kt

@@ -1,13 +1,7 @@
 package com.correx.core.sessions
 
-import com.correx.core.events.events.SessionCompletedEvent
-import com.correx.core.events.events.SessionFailedEvent
-import com.correx.core.events.events.SessionPausedEvent
-import com.correx.core.events.events.SessionResumedEvent
-import com.correx.core.events.events.SessionStartedEvent
 import com.correx.core.events.events.StageCompletedEvent
 import com.correx.core.events.events.StageFailedEvent
-import com.correx.core.events.events.StageStartedEvent
 import com.correx.core.events.events.StoredEvent
 import com.correx.core.events.events.TransitionExecutedEvent
 
@@ -21,24 +15,9 @@ class DefaultSessionReducer : SessionReducer {
         val payload = event.payload
 
         val newStatus = when (payload) {
-
-            is SessionStartedEvent ->
-                SessionStatus.ACTIVE
-
-            is SessionPausedEvent ->
-                SessionStatus.PAUSED
-
-            is SessionResumedEvent ->
-                SessionStatus.ACTIVE
-
-            is SessionCompletedEvent ->
-                SessionStatus.COMPLETED
-
-            is SessionFailedEvent,
             is StageFailedEvent ->
                 SessionStatus.FAILED
 
-            is StageStartedEvent,
             is StageCompletedEvent,
             is TransitionExecutedEvent ->
                 SessionStatus.ACTIVE