fix(hooks): use API instead of filesystem to resolve model info for session.prompt

Previously, continuation hooks (todo-continuation, boulder-continuation, ralph-loop)
and background tasks resolved model info from filesystem cache, which could be stale
or missing. This caused session.prompt to fallback to default model (Sonnet) instead
of using the originally configured model (e.g., Opus).

Now all session.prompt calls first try API (session.messages) to get current model
info, with filesystem as fallback if API fails.

Affected files:
- todo-continuation-enforcer.ts
- sisyphus-orchestrator/index.ts
- ralph-loop/index.ts
- background-agent/manager.ts
- sisyphus-task/tools.ts
- hook-message-injector/index.ts (export ToolPermission type)

src/features/background-agent/manager.ts

@@ -675,21 +675,33 @@ Use \`background_output(task_id="${task.id}")\` to retrieve this result when rea
 </system-reminder>`
     }
 
-    // Dynamically lookup the parent session's current message context
-    // This ensures we use the CURRENT model/agent, not the stale one from task creation time
-    const messageDir = getMessageDir(task.parentSessionID)
-    const currentMessage = messageDir ? findNearestMessageWithFields(messageDir) : null
+    let agent: string | undefined = task.parentAgent
+    let model: { providerID: string; modelID: string } | undefined
 
-    const agent = currentMessage?.agent ?? task.parentAgent
-    const model = currentMessage?.model?.providerID && currentMessage?.model?.modelID
-      ? { providerID: currentMessage.model.providerID, modelID: currentMessage.model.modelID }
-      : undefined
+    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 } }
+      }>
+      for (let i = messages.length - 1; i >= 0; i--) {
+        const info = messages[i].info
+        if (info?.agent || info?.model) {
+          agent = info.agent ?? task.parentAgent
+          model = info.model
+          break
+        }
+      }
+    } catch {
+      const messageDir = getMessageDir(task.parentSessionID)
+      const currentMessage = messageDir ? findNearestMessageWithFields(messageDir) : null
+      agent = currentMessage?.agent ?? task.parentAgent
+      model = currentMessage?.model?.providerID && currentMessage?.model?.modelID
+        ? { providerID: currentMessage.model.providerID, modelID: currentMessage.model.modelID }
+        : undefined
+    }
 
     log("[background-agent] notifyParentSession context:", {
       taskId: task.id,
-      messageDir: !!messageDir,
-      currentAgent: currentMessage?.agent,
-      currentModel: currentMessage?.model,
       resolvedAgent: agent,
       resolvedModel: model,
     })

src/features/hook-message-injector/index.ts

@@ -1,4 +1,4 @@
 export { injectHookMessage, findNearestMessageWithFields, findFirstMessageWithAgent } from "./injector"
 export type { StoredMessage } from "./injector"
-export type { MessageMeta, OriginalMessageContext, TextPart } from "./types"
+export type { MessageMeta, OriginalMessageContext, TextPart, ToolPermission } from "./types"
 export { MESSAGE_STORAGE } from "./constants"