docs: add GPT-5 Codex guidelines for CLI usage
- Added detailed guidelines for GPT-5 Codex in Codex CLI.
- Expanded instructions on sandboxing, approvals, editing constraints, and style requirements.
- Included presentation and response formatting best practices.
fix(codex_instructions): update comparison logic to use prefix matching
- Changed system instructions comparison to use `strings.HasPrefix` for improved flexibility.
