Import only the continuation of backgrounded Claude sessions (#55365)

## Why

Backgrounding a Claude session creates a continuation that includes its history. Offering both sessions for import duplicates that history.

## What changed

Skip sessions containing a `continued-in` record with a nonblank `continuedInSessionId` during import detection, leaving the continuation available for import.

## Testing

Add detection and app-server regression tests covering continuation-only detection, importing the full history into one thread, and no remaining import candidates afterward.

GitOrigin-RevId: bf8383faadce984ae3c41a0b8e8b3072680fd4b2

codex-rs/external-agent-migration/src/detect/sessions/cla.rs

@@ -65,6 +65,7 @@ mod tests {
     use super::*;
     use crate::sessions::ledger::record_imported_session;
     use codex_protocol::ThreadId;
+    use pretty_assertions::assert_eq;
     use serde_json::Value as JsonValue;
     use std::fs::FileTimes;
     use std::fs::OpenOptions;
@@ -101,6 +102,45 @@ mod tests {
         );
     }
 
+    #[test]
+    fn detects_only_the_continuation_of_a_backgrounded_session() {
+        let root = TempDir::new().expect("tempdir");
+        let external_agent_home = root.path().join(".external");
+        let project_root = root.path().join("repo");
+        let original = record("user", "plan a vacation", &project_root);
+        write_session(
+            &external_agent_home,
+            &project_root,
+            "original.jsonl",
+            &[
+                original.clone(),
+                serde_json::json!({
+                    "type": "continued-in",
+                    "sessionId": "original",
+                    "continuedInSessionId": "background",
+                }),
+            ],
+        );
+        let continuation = write_session(
+            &external_agent_home,
+            &project_root,
+            "background.jsonl",
+            &[
+                original,
+                record("assistant", "Here is your itinerary", &project_root),
+            ],
+        );
+
+        assert_eq!(
+            detect_recent_cla_sessions(&external_agent_home, root.path()).expect("detect"),
+            vec![ExternalAgentSessionMigration {
+                path: continuation,
+                cwd: project_root,
+                title: Some("plan a vacation".to_string()),
+            }]
+        );
+    }
+
     #[test]
     fn prefers_latest_custom_title_over_first_user_message() {
         let root = TempDir::new().expect("tempdir");

codex-rs/external-agent-migration/src/sessions/records_cla.rs

@@ -40,6 +40,15 @@ pub fn summarize_session(path: &Path) -> io::Result<Option<SessionSummary>> {
         let Ok(mut record) = serde_json::from_str::<JsonValue>(trimmed) else {
             continue;
         };
+        // Backgrounding replaces this session with a continuation that includes its history.
+        if record.get("type").and_then(JsonValue::as_str) == Some("continued-in")
+            && record
+                .get("continuedInSessionId")
+                .and_then(JsonValue::as_str)
+                .is_some_and(|id| !id.trim().is_empty())
+        {
+            return Ok(None);
+        }
         if cwd.is_none() {
             cwd = record
                 .get("cwd")