Add GPT-5.3 model support and reasoning coverage

[mrbeandev]

Summary

• add GPT-5.3 and GPT-5.3 Codex mappings to model normalization, including preset variants
• extend prompt family selection and instruction-cache file routing for GPT-5.3 model families
• expand request transformer tests for GPT-5.3 normalization and reasoning behavior (none, minimal, and xhigh)
• refresh AGENTS documentation to reflect GPT-5.3 support and caching/model-family behavior

Validation

• npm test
• npm run typecheck
• npm run build