fix(background-agent): gracefully handle aborted parent session in notifyParentSession

When the main session is aborted while background tasks are running,
notifyParentSession() would attempt to call session.messages() and
session.prompt() on the aborted parent session, causing exceptions
that could crash the TUI.

- Add isAbortedSessionError() helper to detect abort-related errors
- Add abort check in session.messages() catch block with early return
- Add abort check in session.prompt() catch block with early return
- Add test case covering aborted parent session scenario

Fixes TUI crash when aborting main session with running background tasks.

src/features/background-agent/manager.ts

@@ -1123,7 +1123,14 @@ Use \`background_output(task_id="${task.id}")\` to retrieve this result when rea
           break
         }
       }
-    } catch {
+    } catch (error) {
+      if (this.isAbortedSessionError(error)) {
+        log("[background-agent] Parent session aborted, skipping notification:", {
+          taskId: task.id,
+          parentSessionID: task.parentSessionID,
+        })
+        return
+      }
       const messageDir = getMessageDir(task.parentSessionID)
       const currentMessage = messageDir ? findNearestMessageWithFields(messageDir) : null
       agent = currentMessage?.agent ?? task.parentAgent
@@ -1154,6 +1161,13 @@ Use \`background_output(task_id="${task.id}")\` to retrieve this result when rea
         noReply: !allComplete,
       })
     } catch (error) {
+      if (this.isAbortedSessionError(error)) {
+        log("[background-agent] Parent session aborted, skipping notification:", {
+          taskId: task.id,
+          parentSessionID: task.parentSessionID,
+        })
+        return
+      }
       log("[background-agent] Failed to send notification:", error)
     }
 
@@ -1192,6 +1206,28 @@ Use \`background_output(task_id="${task.id}")\` to retrieve this result when rea
     return `${seconds}s`
   }
 
+  private isAbortedSessionError(error: unknown): boolean {
+    const message = this.getErrorText(error)
+    return message.toLowerCase().includes("aborted")
+  }
+
+  private getErrorText(error: unknown): string {
+    if (!error) return ""
+    if (typeof error === "string") return error
+    if (error instanceof Error) {
+      return `${error.name}: ${error.message}`
+    }
+    if (typeof error === "object" && error !== null) {
+      if ("message" in error && typeof error.message === "string") {
+        return error.message
+      }
+      if ("name" in error && typeof error.name === "string") {
+        return error.name
+      }
+    }
+    return ""
+  }
+
   private hasRunningTasks(): boolean {
     for (const task of this.tasks.values()) {
       if (task.status === "running") return true