Add a single checkbox next to the existing Custom Instructions box:
☐ Priority
When enabled, the instructions in that box cannot be overridden by a user message. They can only be changed by editing the Custom Instructions in Settings.
When disabled, Custom Instructions behave normally.
Rationale: Users may want certain instructions to function as firm, persistent rules rather than preferences that can be overridden in conversation. Requiring changes to be made through Settings would provide a simple, deliberate mechanism for changing those rules without adding a complex instruction hierarchy.
This could be implemented without adding additional instruction boxes or changing the existing behavior for users who leave the option disabled.