Support Codex subagent transcripts

server/src/session/subagent.rs

@@ -7,9 +7,10 @@
 //! work on a subagent's file unchanged.
 //!
 //! Storage is `<session dir>/subagents/<id>/{meta.json,transcript.jsonl}`,
-//! where `<id>` is the Task tool_use id that started it -- unique, stable
-//! across a backend restart, and already the key the parent side uses. Only
-//! ids matching [`is_subagent_id`] are ever turned into a path.
+//! where `<id>` is Claude's Task tool_use id or Codex's child thread id --
+//! unique, stable across a backend restart, and already the key the parent
+//! side uses. Only ids matching [`is_subagent_id`] are ever turned into a
+//! path.
 
 use std::collections::HashMap;
 use std::fs;
@@ -112,8 +113,8 @@ impl Subagent {
     }
 }
 
-/// Every subagent one session has started, keyed by the Task tool_use id
-/// that names it.
+/// Every subagent one session has started, keyed by the provider's stable
+/// parent-side id for it.
 ///
 /// Lives beside a session's driver rather than inside it: a claude driver
 /// holds an `Arc` to this and routes child lines into it; echo uses it for