feat: config aliases (#18140)

Rename `no_memories_if_mcp_or_web_search` →
`disable_on_external_context` with backward compatibility

While doing so, we add a key alias system on our layer merging system.
What we try to avoid is a case where a company managed config use an old
name while the user has a new name in it's local config (which would
make the deserialization fail)

codex-rs/tui/src/app.rs

@@ -8274,8 +8274,9 @@ mod tests {
             Some(&TomlValue::Boolean(false))
         );
         assert!(
-            !memories.contains_key("no_memories_if_mcp_or_web_search"),
-            "the TUI menu should not write the MCP pollution setting"
+            !memories.contains_key("disable_on_external_context")
+                && !memories.contains_key("no_memories_if_mcp_or_web_search"),
+            "the TUI menu should not write the external-context memory setting"
         );
         app_server.shutdown().await?;
         Ok(())