feat(sisyphus-task): inherit parent model for categories and show fallback warning

- Change model priority: user override > parent model > category default
- Add ModelFallbackInfo to track model resolution type
- Show warning toast when category uses inherited or default model
- Add tests for model fallback info in task toast

src/agents/sisyphus-junior.ts

@@ -84,13 +84,14 @@ export const SISYPHUS_JUNIOR_DEFAULTS = {
 } as const
 
 export function createSisyphusJuniorAgentWithOverrides(
-  override: AgentOverrideConfig | undefined
+  override: AgentOverrideConfig | undefined,
+  systemDefaultModel?: string
 ): AgentConfig {
   if (override?.disable) {
     override = undefined
   }
 
-  const model = override?.model ?? SISYPHUS_JUNIOR_DEFAULTS.model
+  const model = override?.model ?? systemDefaultModel ?? SISYPHUS_JUNIOR_DEFAULTS.model
   const temperature = override?.temperature ?? SISYPHUS_JUNIOR_DEFAULTS.temperature
 
   const promptAppend = override?.prompt_append

src/agents/utils.ts

@@ -192,7 +192,7 @@ export function createBuiltinAgents(
 
   if (!disabledAgents.includes("orchestrator-sisyphus")) {
     const orchestratorOverride = agentOverrides["orchestrator-sisyphus"]
-    const orchestratorModel = orchestratorOverride?.model
+    const orchestratorModel = orchestratorOverride?.model ?? systemDefaultModel
     let orchestratorConfig = createOrchestratorSisyphusAgent({
       model: orchestratorModel,
       availableAgents,