core: reuse parent shell snapshot for thread-spawn subagents (#13052)

## Summary
- reuse the parent shell snapshot when spawning/forking/resuming
`SessionSource::SubAgent(SubAgentSource::ThreadSpawn { .. })` sessions
- plumb inherited snapshot through `AgentControl -> ThreadManager ->
Codex::spawn -> SessionConfiguration`
- skip shell snapshot refresh on cwd updates for thread-spawn subagents
so inherited snapshots are not replaced

## Why
- avoids per-subagent shell snapshot creation and cleanup work
- keeps thread-spawn subagents on the parent snapshot path, matching the
intended parent/child snapshot model

## Validation
- `just fmt` (in `codex-rs`)
- `cargo test -p codex-core --no-run`
- `cargo test -p codex-core spawn_agent -- --nocapture`
- `cargo test -p codex-core --test all
suite::agent_jobs::spawn_agents_on_csv_runs_and_exports`

## Notes
- full `cargo test -p codex-core --test all` was left running separately
for broader verification

Co-authored-by: Codex <noreply@openai.com>

codex-rs/core/src/agent/control.rs

@@ -7,6 +7,7 @@ use crate::find_thread_path_by_id_str;
 use crate::rollout::RolloutRecorder;
 use crate::session_prefix::format_subagent_context_line;
 use crate::session_prefix::format_subagent_notification_message;
+use crate::shell_snapshot::ShellSnapshot;
 use crate::state_db;
 use crate::thread_manager::ThreadManagerState;
 use codex_protocol::ThreadId;
@@ -83,6 +84,9 @@ impl AgentControl {
     ) -> CodexResult<ThreadId> {
         let state = self.upgrade()?;
         let mut reservation = self.state.reserve_spawn_slot(config.agent_max_threads)?;
+        let inherited_shell_snapshot = self
+            .inherited_shell_snapshot_for_source(&state, session_source.as_ref())
+            .await;
         let session_source = match session_source {
             Some(SessionSource::SubAgent(SubAgentSource::ThreadSpawn {
                 parent_thread_id,
@@ -161,6 +165,7 @@ impl AgentControl {
                             self.clone(),
                             session_source,
                             false,
+                            inherited_shell_snapshot,
                         )
                         .await?
                 } else {
@@ -171,6 +176,7 @@ impl AgentControl {
                             session_source,
                             false,
                             None,
+                            inherited_shell_snapshot,
                         )
                         .await?
                 }
@@ -235,6 +241,9 @@ impl AgentControl {
             other => other,
         };
         let notification_source = session_source.clone();
+        let inherited_shell_snapshot = self
+            .inherited_shell_snapshot_for_source(&state, Some(&session_source))
+            .await;
         let rollout_path =
             find_thread_path_by_id_str(config.codex_home.as_path(), &thread_id.to_string())
                 .await?
@@ -246,6 +255,7 @@ impl AgentControl {
                 rollout_path,
                 self.clone(),
                 session_source,
+                inherited_shell_snapshot,
             )
             .await?;
         reservation.commit(resumed_thread.thread_id);
@@ -431,6 +441,22 @@ impl AgentControl {
             .upgrade()
             .ok_or_else(|| CodexErr::UnsupportedOperation("thread manager dropped".to_string()))
     }
+
+    async fn inherited_shell_snapshot_for_source(
+        &self,
+        state: &Arc<ThreadManagerState>,
+        session_source: Option<&SessionSource>,
+    ) -> Option<Arc<ShellSnapshot>> {
+        let Some(SessionSource::SubAgent(SubAgentSource::ThreadSpawn {
+            parent_thread_id, ..
+        })) = session_source
+        else {
+            return None;
+        };
+
+        let parent_thread = state.get_thread(*parent_thread_id).await.ok()?;
+        parent_thread.codex.session.user_shell().shell_snapshot()
+    }
 }
 #[cfg(test)]
 mod tests {