fix(sisyphus-task): use SDK session.messages API for parent agent lookup

Background task notifications were showing 'build' agent instead of the
actual parent agent (e.g., 'Sisyphus'). The hook-injected message storage
only contains limited info; the actual agent name is in the SDK session.

Changes:
- Add getParentAgentFromSdk() to query SDK messages API
- Look up agent from SDK first, fallback to hook-injected messages
- Ensures background tasks correctly preserve parent agent context

src/tools/sisyphus-task/tools.ts

@@ -38,6 +38,19 @@ function getMessageDir(sessionID: string): string | null {
   return null
 }
 
+async function getParentAgentFromSdk(client: OpencodeClient, sessionID: string): Promise<string | undefined> {
+  try {
+    const messagesResp = await client.session.messages({ path: { id: sessionID } })
+    const messages = (messagesResp.data ?? []) as Array<{ info?: { agent?: string } }>
+    for (let i = messages.length - 1; i >= 0; i--) {
+      if (messages[i].info?.agent) {
+        return messages[i].info?.agent
+      }
+    }
+  } catch {}
+  return undefined
+}
+
 function formatDuration(start: Date, end?: Date): string {
   const duration = (end ?? new Date()).getTime() - start.getTime()
   const seconds = Math.floor(duration / 1000)
@@ -144,13 +157,14 @@ export function createSisyphusTask(options: SisyphusTaskToolOptions): ToolDefini
         skillContent = Array.from(resolved.values()).join("\n\n")
       }
 
+      const sdkParentAgent = await getParentAgentFromSdk(client, ctx.sessionID)
       const messageDir = getMessageDir(ctx.sessionID)
       const prevMessage = messageDir ? findNearestMessageWithFields(messageDir) : null
+      const parentAgent = sdkParentAgent ?? prevMessage?.agent
       const parentModel = prevMessage?.model?.providerID && prevMessage?.model?.modelID
         ? { providerID: prevMessage.model.providerID, modelID: prevMessage.model.modelID }
         : undefined
 
-      // Handle resume case first
       if (args.resume) {
         if (runInBackground) {
           try {
@@ -160,7 +174,7 @@ export function createSisyphusTask(options: SisyphusTaskToolOptions): ToolDefini
               parentSessionID: ctx.sessionID,
               parentMessageID: ctx.messageID,
               parentModel,
-              parentAgent: prevMessage?.agent,
+              parentAgent,
             })
 
             ctx.metadata?.({
@@ -327,7 +341,7 @@ ${textContent || "(No text output)"}`
             parentSessionID: ctx.sessionID,
             parentMessageID: ctx.messageID,
             parentModel,
-            parentAgent: prevMessage?.agent,
+            parentAgent,
             model: categoryModel,
             skills: args.skills,
             skillContent: systemContent,