feat: rename experimental_instructions_file to model_instructions_file (#9555)

A user who has `experimental_instructions_file` set will now see this:

<img width="888" height="660" alt="image"
src="https://github.com/user-attachments/assets/51c98312-eb9b-4881-81f1-bea6677e158d"
/>

And a `codex exec` would include this warning:

<img width="888" height="660" alt="image"
src="https://github.com/user-attachments/assets/a89f62be-1edf-4593-a75e-e0b4a762ed7d"
/>

codex-rs/core/config.schema.json

@@ -53,14 +53,6 @@
     "experimental_compact_prompt_file": {
       "$ref": "#/definitions/AbsolutePathBuf"
     },
-    "experimental_instructions_file": {
-      "description": "Legacy, now use features",
-      "allOf": [
-        {
-          "$ref": "#/definitions/AbsolutePathBuf"
-        }
-      ]
-    },
     "experimental_use_freeform_apply_patch": {
       "type": "boolean"
     },
@@ -240,6 +232,14 @@
       "type": "integer",
       "format": "int64"
     },
+    "model_instructions_file": {
+      "description": "Optional path to a file containing model instructions that will override the built-in instructions for the selected model. Users are STRONGLY DISCOURAGED from using this field, as deviating from the instructions sanctioned by Codex will likely degrade model performance.",
+      "allOf": [
+        {
+          "$ref": "#/definitions/AbsolutePathBuf"
+        }
+      ]
+    },
     "model_personality": {
       "description": "EXPERIMENTAL Optionally specify a personality for the model",
       "allOf": [
@@ -545,9 +545,6 @@
         "experimental_compact_prompt_file": {
           "$ref": "#/definitions/AbsolutePathBuf"
         },
-        "experimental_instructions_file": {
-          "$ref": "#/definitions/AbsolutePathBuf"
-        },
         "experimental_use_freeform_apply_patch": {
           "type": "boolean"
         },
@@ -640,6 +637,14 @@
         "model": {
           "type": "string"
         },
+        "model_instructions_file": {
+          "description": "Optional path to a file containing model instructions.",
+          "allOf": [
+            {
+              "$ref": "#/definitions/AbsolutePathBuf"
+            }
+          ]
+        },
         "model_personality": {
           "$ref": "#/definitions/Personality"
         },