fix(exec): pass undefined instead of null for optional approval params

TypeBox Type.Optional(Type.String()) accepts string|undefined but NOT null.
Discord exec was failing with 'resolvedPath must be string' because callers
passed null explicitly. Web UI worked because it skipped the approval request.

Fixes exec approval validation error in Discord-triggered sessions.

src/cli/nodes-cli/register.invoke.ts

@@ -267,8 +267,8 @@ export function registerNodesInvokeCommands(nodes: Command) {
               security: hostSecurity,
               ask: hostAsk,
               agentId,
-              resolvedPath: null,
-              sessionKey: null,
+              resolvedPath: undefined,
+              sessionKey: undefined,
               timeoutMs: 120_000,
             })) as { decision?: string } | null;
             const decision =