refactor(sisyphus-task): use dynamic model fallback from OpenCode config

- Remove hardcoded "anthropic/claude-sonnet-4-5" fallback
- Fetch systemDefaultModel from client.config.get() at tool boundary
- Add 'category-default' and 'system-default' fallback types
- Use switch(actualModel) for cleaner type detection
- Add guard clauses and fail-loud validation for invalid models
- Wrap config fetch in try/catch for graceful degradation
- Update toast messages with typed suffixMap

src/features/task-toast-manager/manager.ts

@@ -110,7 +110,12 @@ export class TaskToastManager {
     // Show model fallback warning for the new task if applicable
     if (newTask.modelInfo && newTask.modelInfo.type !== "user-defined") {
       const icon = "⚠️"
-      const suffix = newTask.modelInfo.type === "inherited" ? " (inherited)" : " (default)"
+      const suffixMap: Partial<Record<ModelFallbackInfo["type"], string>> = {
+        inherited: " (inherited)",
+        "category-default": " (category default)",
+        "system-default": " (system default)",
+      }
+      const suffix = suffixMap[newTask.modelInfo.type] ?? ""
       lines.push(`${icon} Model: ${newTask.modelInfo.model}${suffix}`)
       lines.push("")
     }