feat: implement model-specific Codex instructions for GPT-5

- 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.

internal/misc/codex_instructions.go

@@ -9,5 +9,15 @@ import _ "embed"
 // which is embedded into the application binary at compile time. This variable
 // contains instructional text used for Codex-related operations and model guidance.
 //
-//go:embed codex_instructions.txt
-var CodexInstructions string
+//go:embed gpt_5_instructions.txt
+var GPT5Instructions string
+
+//go:embed gpt_5_codex_instructions.txt
+var GPT5CodexInstructions string
+
+func CodexInstructions(modelName string) string {
+	if modelName == "gpt-5-codex" {
+		return GPT5CodexInstructions
+	}
+	return GPT5Instructions
+}