fix(hooks): extract model from assistant messages with flat modelID/providerID

OpenCode API returns different structures for user vs assistant messages:
- User: info.model = { providerID, modelID } (nested)
- Assistant: info.modelID, info.providerID (flat top-level)

Previous code only checked nested format, causing model info loss when
continuation hooks fired after assistant messages.

Files modified:
- todo-continuation-enforcer.ts
- ralph-loop/index.ts
- sisyphus-task/tools.ts
- background-agent/manager.ts

Added test for assistant message model extraction.

src/hooks/todo-continuation-enforcer.ts

@@ -381,15 +381,17 @@ export function createTodoContinuationEnforcer(
           info?: {
             agent?: string
             model?: { providerID: string; modelID: string }
+            modelID?: string
+            providerID?: string
             tools?: Record<string, ToolPermission>
           }
         }>
         for (let i = messages.length - 1; i >= 0; i--) {
           const info = messages[i].info
-          if (info?.agent || info?.model) {
+          if (info?.agent || info?.model || (info?.modelID && info?.providerID)) {
             resolvedInfo = {
               agent: info.agent,
-              model: info.model,
+              model: info.model ?? (info.providerID && info.modelID ? { providerID: info.providerID, modelID: info.modelID } : undefined),
               tools: info.tools,
             }
             break