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/features/background-agent/manager.ts

@@ -826,13 +826,13 @@ Use \`background_output(task_id="${task.id}")\` to retrieve this result when rea
     try {
       const messagesResp = await this.client.session.messages({ path: { id: task.parentSessionID } })
       const messages = (messagesResp.data ?? []) as Array<{
-        info?: { agent?: string; model?: { providerID: string; modelID: string } }
+        info?: { agent?: string; model?: { providerID: string; modelID: string }; modelID?: string; providerID?: string }
       }>
       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)) {
           agent = info.agent ?? task.parentAgent
-          model = info.model
+          model = info.model ?? (info.providerID && info.modelID ? { providerID: info.providerID, modelID: info.modelID } : undefined)
           break
         }
       }