fix: use dynamic message lookup for model/agent context in prompts

Instead of using stale parentModel/parentAgent from task state, now
dynamically looks up the current message to get fresh model/agent values.
Applied across all prompt injection points:
- background-agent notifyParentSession
- ralph-loop continuation
- sisyphus-orchestrator boulder continuation
- sisyphus-task resume

🤖 Generated with [OhMyOpenCode](https://github.com/code-yeongyu/oh-my-opencode) assistance

src/hooks/sisyphus-orchestrator/index.ts

@@ -407,10 +407,17 @@ export function createSisyphusOrchestratorHook(
     try {
       log(`[${HOOK_NAME}] Injecting boulder continuation`, { sessionID, planName, remaining })
 
+      const messageDir = getMessageDir(sessionID)
+      const currentMessage = messageDir ? findNearestMessageWithFields(messageDir) : null
+      const model = currentMessage?.model?.providerID && currentMessage?.model?.modelID
+        ? { providerID: currentMessage.model.providerID, modelID: currentMessage.model.modelID }
+        : undefined
+
       await ctx.client.session.prompt({
         path: { id: sessionID },
         body: {
           agent: "orchestrator-sisyphus",
+          ...(model !== undefined ? { model } : {}),
           parts: [{ type: "text", text: prompt }],
         },
         query: { directory: ctx.directory },