[agent] add "custom_behavior" section to the prompt

### StatGPT Backend version

latest

### What is the problem this feature will solve?

- there is no dedicated section in agent's system prompt to customize its behavior
- currently we use "language_instructions" section. this is incorrect, because:
- the section name is "Language instructions"
- the section has following prefix: "Follow this **language guidance** strictly:"
- language instructions are shared between supreme agent and guardrails
- we also use "additional_context" section for that. this is incorrect because:
- this section has lower priority - it's observed that agent follows these instructions less strictly
- this section was not meant for instructions, but purely for the context

### What is the proposed feature or solution?

add "custom_behavior" section (name could be different).

### What alternatives have you considered?

_No response_

No file or test is named. Start by locating where the agent system prompt is assembled and compare the existing language_instructions and additional_context sections. Done means the prompt has a dedicated behavior-instructions section, separate from language guidance and context, with the intended instruction priority clarified.

Written by the indexing model from the issue text.