Source multi-agent instructions from the model catalog (#38619)

## What changed

- Add model-catalog messages for root and subagent roles, explicit delegation, and delegation hints.
- Resolve role instructions in config, catalog, then bundled-default order, while preserving empty values as an explicit way to suppress fallback text.
- Refresh catalog-provided role and mode instructions when the model changes, and give full-history forks the selected child model's subagent role without retaining the parent's role guidance.
- Keep existing config overrides and reasoning-effort behavior, including proactive delegation for ultra reasoning effort.

## Testing

- Cover message deserialization and preservation through model overrides.
- Cover precedence, empty overrides, model switches, resumed sessions, and full-history subagent forks.

GitOrigin-RevId: 4625cf7c6a5490176adddfaa0fb99100707daea9

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

@@ -22,6 +22,7 @@ use crate::config::ManagedFeatures;
 use crate::config::resolve_tool_suggest_config_from_layer_stack;
 use crate::context::ContextualUserFragment;
 use crate::context::ModelSwitchInstructions;
+use crate::context::MultiAgentRoleInstructions;
 use crate::context::NetworkRuleSaved;
 use crate::context::RecommendedPluginsInstructions;
 use crate::context::world_state::WorldState;
@@ -3603,6 +3604,11 @@ impl Session {
                 "developer" if fragment.markers().0 == MULTI_AGENT_MODE_OPEN_TAG => {
                     initial_multi_agent_mode = Some(fragment);
                 }
+                "developer"
+                    if fragment.markers().0 == MultiAgentRoleInstructions::type_markers().0 =>
+                {
+                    separate_developer_sections.push(fragment.render());
+                }
                 "developer"
                     if fragment.requires_separate_message() && fragment.markers().0.is_empty() =>
                 {