feat(background-agent): handle "interrupt" in notifications, output, and formatting

Update notification systems to display INTERRUPTED status.

Add interrupt handling to background_output tool (terminal status).

Add interrupt-specific status note to formatTaskStatus.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-opencode)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

src/tools/background-task/task-status-format.ts

@@ -32,20 +32,24 @@ ${truncated}
 \`\`\``
   }
 
-  let statusNote = ""
-  if (task.status === "pending") {
-    statusNote = `
+   let statusNote = ""
+   if (task.status === "pending") {
+     statusNote = `
 
 > **Queued**: Task is waiting for a concurrency slot to become available.`
-  } else if (task.status === "running") {
-    statusNote = `
+   } else if (task.status === "running") {
+     statusNote = `
 
 > **Note**: No need to wait explicitly - the system will notify you when this task completes.`
-  } else if (task.status === "error") {
-    statusNote = `
+   } else if (task.status === "error") {
+     statusNote = `
 
 > **Failed**: The task encountered an error. Check the last message for details.`
-  }
+   } else if (task.status === "interrupt") {
+     statusNote = `
+
+> **Interrupted**: The task was interrupted by a prompt error. The session may contain partial results.`
+   }
 
   const durationLabel = task.status === "pending" ? "Queued for" : "Duration"