feat: guardian as an extension (contributors part) (#22216)

Part 1 of guardian as extension. This bind all the logic to spawn
another agent from an extension and it adds `ThreadId` in the start
thread collaborator

codex-rs/core/src/session/session.rs

@@ -815,6 +815,7 @@ impl Session {
             let thread_extension_data = codex_extension_api::ExtensionData::new();
             for contributor in extensions.thread_start_contributors() {
                 contributor.contribute(
+                    thread_id,
                     config.as_ref(),
                     &session_extension_data,
                     &thread_extension_data,

codex-rs/core/src/thread_manager.rs

@@ -567,6 +567,36 @@ impl ThreadManager {
         .await
     }
 
+    // TODO(jif) merge with fork_agent
+    /// Spawn a subagent by forking persisted history from `forked_from_thread_id`.
+    pub async fn spawn_subagent(
+        &self,
+        forked_from_thread_id: ThreadId,
+        mut options: StartThreadOptions,
+    ) -> CodexResult<NewThread> {
+        let fork_source = self.get_thread(forked_from_thread_id).await?;
+        // Persist queued rollout updates before reading the fork snapshot.
+        fork_source.ensure_rollout_materialized().await;
+        fork_source.flush_rollout().await?;
+        let stored_thread = fork_source
+            .read_thread(
+                /*include_archived*/ true, /*include_history*/ true,
+            )
+            .await
+            .map_err(|err| {
+                CodexErr::Fatal(format!(
+                    "failed to read subagent fork source {forked_from_thread_id}: {err}"
+                ))
+            })?;
+        let history = stored_thread_to_initial_history(stored_thread, fork_source.rollout_path())?;
+        options.initial_history = fork_history_from_snapshot(
+            ForkSnapshot::Interrupted,
+            history,
+            InterruptedTurnHistoryMarker::from_config(&options.config),
+        );
+        self.start_thread_with_options(options).await
+    }
+
     pub async fn resume_thread_from_rollout(
         &self,
         config: Config,