[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/tools/src/tool_config_tests.rs

@@ -27,6 +27,7 @@ fn model_with_shell_type(shell_type: ConfigShellToolType) -> ModelInfo {
         upgrade: None,
         base_instructions: String::new(),
         model_messages: None,
+        include_skills_usage_instructions: false,
         supports_reasoning_summaries: false,
         default_reasoning_summary: Default::default(),
         support_verbosity: false,