[codex] Use model metadata for skills usage instructions (#29740)

## Summary

- add a false-by-default `include_skills_usage_instructions` model
metadata field
- enable the field for the bundled `gpt-5.5` model metadata
- consume the metadata in both core and extension skill rendering
- remove hardcoded legacy-model matching and its marker plumbing

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

@@ -701,6 +701,9 @@ impl Session {
                 &per_turn_config.to_models_manager_config(),
             )
             .await;
+        self.services
+            .thread_extension_data
+            .insert(model_info.clone());
 
         let multi_agent_version = match multi_agent_runtime {
             TurnMultiAgentRuntime::ResolveAndStore => {