fix(agents): model rehearsal modes as discriminants

src/commands/agents.db-rehearsal.ts

@@ -68,14 +68,16 @@ type InventoryRequest = {
   configPath: string;
 };
 
-type RehearsalRequest = {
+type RehearsalRequestBase = {
   schemaVersion: 1;
-  mode: "migrate" | "read-only";
   privateStateRoot: string;
   agents: AgentDatabaseRequest[];
   pluginPersistence: PluginPersistenceDeclaration[];
 };
 
+type RehearsalRequest = RehearsalRequestBase &
+  ({ mode: "migrate" } | { mode: "read-only" });
+
 type ParsedRequest = InventoryRequest | RehearsalRequest;
 
 type DatabaseSnapshot = {