Track context windows per agent (#37347)

## Why

Forked subagents can inherit a parent's compacted history, but their context-window metadata needs to identify the child and start a distinct window lineage.

## What changed

- Always identify context windows by agent name and remove the `features.token_budget.mode` setting.
- Persist the agent path in world state and emit context-window metadata as a standalone developer message when the identity changes.
- Reset inherited compaction metadata to the forked subagent's initial context window and restore that window in session state.

## Testing

Extend the full-history subagent test to verify parent and child world-state snapshots, agent identities, distinct window IDs, and the reset child checkpoint.

GitOrigin-RevId: 6584201e57b3d2fba0ce626b89a8b3b0b3c752db

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

@@ -595,6 +595,20 @@ impl Session {
             }
         });
         let initial_auto_compact_window_ids = AutoCompactWindowIds::new_initial();
+        let restore_child_window = matches!(&initial_history, InitialHistory::Forked(_))
+            && session_configuration.session_source.is_non_root_agent()
+            && config.features.enabled(Feature::TokenBudget);
+        if restore_child_window && let InitialHistory::Forked(items) = &mut initial_history {
+            let child_window_id = initial_auto_compact_window_ids.window_id.to_string();
+            for item in items {
+                if let RolloutItem::Compacted(checkpoint) = item {
+                    checkpoint.window_number = Some(0);
+                    checkpoint.first_window_id = Some(child_window_id.clone());
+                    checkpoint.previous_window_id = None;
+                    checkpoint.window_id = Some(child_window_id.clone());
+                }
+            }
+        }
         let agent_control = agent_control.with_session_id(
             session_id,
             config
@@ -1313,6 +1327,12 @@ impl Session {
 
             // record_initial_history can emit events. We record only after the SessionConfiguredEvent is emitted.
             Box::pin(sess.record_initial_history(initial_history)).await;
+            if restore_child_window {
+                sess.state.lock().await.restore_auto_compact_window(
+                    /*window_number*/ 0,
+                    initial_auto_compact_window_ids,
+                );
+            }
             if matches!(&sess.fork_persistence, ForkPersistence::Referenced { .. }) {
                 // Keep the source reserved until the child's history reference is durable.
                 sess.try_ensure_rollout_materialized().await?;