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/models-manager/src/model_info.rs

@@ -57,6 +57,7 @@ pub fn with_config_overrides(mut model: ModelInfo, config: &ModelsManagerConfig)
             collaboration_modes: None,
             auto_review: None,
             permissions: None,
+            multi_agent: None,
             token_budget: None,
         });
         model_messages.instructions_template = Some(base_instructions.clone());
@@ -198,6 +199,7 @@ fn local_model_messages_for_slug(slug: &str) -> ModelMessages {
             collaboration_modes: None,
             auto_review: None,
             permissions: None,
+            multi_agent: None,
             token_budget: None,
         },
         _ => ModelMessages {
@@ -207,6 +209,7 @@ fn local_model_messages_for_slug(slug: &str) -> ModelMessages {
             collaboration_modes: None,
             auto_review: None,
             permissions: None,
+            multi_agent: None,
             token_budget: None,
         },
     }