chore(config) personality as a feature (#10116)

## Summary
Sets up an explicit Feature flag for `/personality`, so users can now
opt in to it via `/experimental`. #10114 also updates the config

## Testing
- [x] Tested locally

codex-rs/core/config.schema.json

@@ -180,6 +180,9 @@
             "include_apply_patch_tool": {
               "type": "boolean"
             },
+            "personality": {
+              "type": "boolean"
+            },
             "powershell_utf8": {
               "type": "boolean"
             },
@@ -1198,6 +1201,9 @@
         "include_apply_patch_tool": {
           "type": "boolean"
         },
+        "personality": {
+          "type": "boolean"
+        },
         "powershell_utf8": {
           "type": "boolean"
         },