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.
- Added comprehensive instructions for Codex CLI harness, sandboxing, approvals, and editing constraints to `internal/misc/codex_instructions/`.
- Clarified `approval_policy` configurations and scenarios requiring escalated permissions.
- Provided detailed style and structure guidelines for presenting results in the Codex CLI.
- Added `CodexInstructions(modelName string)` function to dynamically select instructions based on the model (e.g., GPT-5 Codex).
- Introduced `gpt_5_instructions.txt` and `gpt_5_codex_instructions.txt` for respective model configurations.
- Updated translators to pass `modelName` and use the new instruction logic.
