Support Codex subagent transcripts

PLAN.md

@@ -270,6 +270,17 @@ also makes adoption safe: if a backend restart falls between the deltas and the
 completion, the correction is still meaningful without process-local memory
 of which item ids streamed.
 
+**Codex subagents share that one app-server process** (2026-09-13). Every
+thread in the session tree is multiplexed onto its stdout and identified by
+the notification's `threadId`. `collabAgentToolCall` carries the spawn prompt;
+`subAgentActivity` carries the child thread id, path and lifecycle. The driver
+uses that child thread id as the existing `Subagents` registry key, routes the
+child's ordinary items through a separate translator into its own transcript,
+and keeps only the root thread's process state in `CodexDriver`. A child turn
+ending does not finish the child: `completed` or `interrupted` activity does.
+An `agentMessage` marked `delivery: async` is a peer message delivered to a
+thread, never that thread's own assistant reply.
+
 ### The llama driver
 
 One `llama-server` per session, started through the same `Transport` as any
@@ -877,13 +888,12 @@ session, which is what auto-resume needs.
 model, with no process and no controls of its own.** Full design and wire
 shape in `SUBAGENTS.md`, kept separate because the app half is being built
 against it in parallel and it is the shared contract between the two. The
-one-paragraph reason: a session's Task-tool helpers already speak the common
-event model on the parent's own stdout (each line carrying
-`parent_tool_use_id`), so giving each one its own small transcript — same
-file format, same paging routes, same SSE stream, reused by addressing rather
-than by copying — costs a routing step in the translator and a registry
-(`session/subagent.rs`) rather than a second session type with a driver, a
-process and a config entry it does not need.
+one-paragraph reason: Claude's Task helpers and Codex's collaboration threads
+already speak their parent's event stream with a child identifier, so giving
+each one its own small transcript — same file format, same paging routes, same
+SSE stream, reused by addressing rather than by copying — costs a routing step
+in the translator and a registry (`session/subagent.rs`) rather than a second
+session type with a driver, a process and a config entry it does not need.
 
 ### HTTP surface