clarify model_catalog_json only applied on startup (#12379)

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codex-rs/core/src/config/mod.rs

@@ -1109,8 +1109,8 @@ pub struct ConfigToml {
     /// Override to force-enable reasoning summaries for the configured model.
     pub model_supports_reasoning_summaries: Option<bool>,
 
-    /// Optional path to a JSON file containing a complete model catalog.
-    /// When set, this replaces the bundled catalog for this process.
+    /// Optional path to a JSON model catalog (applied on startup only).
+    /// Per-thread `config` overrides are accepted but do not reapply this (no-ops).
     pub model_catalog_json: Option<AbsolutePathBuf>,
 
     /// Optionally specify a personality for the model