fix(start-work): prevent overwriting session agent if already set; inherit parent model for subagent types (#1201)

* fix(start-work): prevent overwriting session agent if already set; inherit parent model for subagent types

* fix(model): include variant in StoredMessage model structure for better context propagation

* fix(injector): include variant in model structure for hook message injection

src/features/builtin-commands/commands.ts

@@ -55,7 +55,6 @@ ${REFACTOR_TEMPLATE}
   },
   "start-work": {
     description: "(builtin) Start Sisyphus work session from Prometheus plan",
-    agent: "atlas",
     template: `<command-instruction>
 ${START_WORK_TEMPLATE}
 </command-instruction>