External agent session support (#19895)

## Summary

This extends external agent detection/import beyond config artifacts so
Codex can detect recent sessions files from the external agent home and
import them into Codex rollout history.

## What changed

- Added a focused `external_agent_sessions` module for:
  - session discovery
  - source-record parsing
  - rollout construction
  - import ledger tracking
- Wired session detection/import into the app-server external agent
config API.
- Added compaction handling so large imported sessions can be resumed
safely before the first follow-up turn.

## Testing

Added coverage for:
- recent-session detection
- custom-title handling
- recency filtering
- dedupe and re-detect-after-source-change behavior
- visible imported turn construction
- backward-compatible import payload deserialization
- end-to-end RPC import flow
- rejection of undetected session paths
- repeat-import behavior
- large-session compaction before first follow-up

Ran:
- `cargo test -p codex-app-server external_agent_config_import_ --test
all`

codex-rs/app-server/src/message_processor.rs

@@ -1138,11 +1138,21 @@ impl MessageProcessor {
                 ExternalAgentConfigMigrationItemType::Plugins
             )
         });
-
+        let pending_session_imports = self
+            .external_agent_config_api
+            .prepare_pending_session_imports(&params)?;
         let pending_plugin_imports = self.external_agent_config_api.import(params).await?;
         if has_plugin_imports {
             self.handle_config_mutation().await;
         }
+        for pending_session_import in pending_session_imports {
+            let imported_thread_id = self
+                .codex_message_processor
+                .import_external_agent_session(pending_session_import.session)
+                .await?;
+            self.external_agent_config_api
+                .record_imported_session(&pending_session_import.source_path, imported_thread_id);
+        }
         self.outgoing
             .send_response(request_id, ExternalAgentConfigImportResponse {})
             .await;