[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/models-manager/src/model_info.rs

@@ -82,6 +82,7 @@ pub fn model_info_from_slug(slug: &str) -> ModelInfo {
         upgrade: None,
         base_instructions: BASE_INSTRUCTIONS.to_string(),
         model_messages: local_personality_messages_for_slug(slug),
+        include_skills_usage_instructions: false,
         supports_reasoning_summaries: false,
         default_reasoning_summary: ReasoningSummary::Auto,
         support_verbosity: false,