refactor(client): trust narrowed surface event markers

The in-operator narrows SessionEvent to events whose surfaceOp is required. Remove redundant undefined checks from both target Definitions; retain append-versus-replacement discrimination and shared prompt interpretation. This deletes dead conditions reported by the full type-aware CI lint gate without adding fallback behavior.

Validation: both Chat and Trajectory definition regression suites pass with the simplified predicates.

packages/client/ui-chat/src/client/conversation-nodes/request-prompt.ts

@@ -66,7 +66,7 @@ export function systemMessageDefinition(inspect: SystemPromptInspector): Convers
     kind: 'system-message',
     target: 'chat',
     match: event => event.type === 'system/message'
-      || ('surfaceOp' in event && event.surfaceOp !== undefined && event.surfaceOp !== 'append')
+      || ('surfaceOp' in event && event.surfaceOp !== 'append')
       ? { id: String(event.seq), role: 'start' }
       : null,
     start: (_context, match, reader) => {

packages/client/ui-trajectory/src/client/trajectory-request-header-definition.ts

@@ -34,7 +34,7 @@ function trajectorySystemMessageDefinition(inspect: SystemPromptInspector): Conv
     kind: 'trajectory-system-message',
     target: 'trajectory',
     match: event => event.type === 'system/message'
-      || ('surfaceOp' in event && event.surfaceOp !== undefined && event.surfaceOp !== 'append')
+      || ('surfaceOp' in event && event.surfaceOp !== 'append')
       ? { id: String(event.seq), role: 'start' }
       : null,
     start: (_context, match, reader) => {