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/mod.rs

@@ -3542,18 +3542,11 @@ impl Session {
                         .join("\n");
                     (!text.is_empty()).then_some(text)
                 });
-            developer_sections.push(
+            separate_developer_sections.push(
                 crate::context::TokenBudgetContext::new(
-                    self.thread_id(),
                     session_source
                         .get_agent_path()
                         .unwrap_or_else(codex_protocol::AgentPath::root),
-                    turn_context
-                        .config
-                        .token_budget
-                        .as_ref()
-                        .map(|config| config.mode)
-                        .unwrap_or_default(),
                     auto_compact_window_ids.first_window_id,
                     auto_compact_window_ids.previous_window_id,
                     auto_compact_window_ids.window_id,