Warn users on enabling underdevelopment features (#9954)

<img width="938" height="73" alt="image"
src="https://github.com/user-attachments/assets/a2d5ac46-92c5-4828-b35e-0965c30cdf36"
/>

codex-rs/core/config.schema.json

@@ -1465,6 +1465,10 @@
       ],
       "description": "User-level skill config entries keyed by SKILL.md path."
     },
+    "suppress_unstable_features_warning": {
+      "description": "Suppress warnings about unstable (under development) features.",
+      "type": "boolean"
+    },
     "tool_output_token_limit": {
       "description": "Token budget applied when storing tool/function outputs in the context manager.",
       "format": "uint",