[FR] Allow config global custom instruction

## Problem Description
Currently, users need to repeatedly specify custom instructions (e.g., response rules, tone, style, and language requirements) for different conversation sessions. This redundancy makes it inefficient to maintain consistent interaction patterns across multiple conversations.

## Proposed Solution
Introduce a `custom_instruction` (like the feature in ChatGPT website) configuration parameter that allows users to define global preset rules in configuration files. This would enable:
```json
{
"default_model": "gpt-4o",
"openai_api_key": "sk-xxxxxxxxxxxxxx",
"custom_instruction": "Remember your name is Jarvis, and I am your master named Kortin. Respond in Simplified Chinese ONLY. Maintain formal tone and avoid emojis.",
}
```
When ask to LLM, the messages will be:
```javascript
[
{
role: 'system',
content: 'Remember your name is Jarvis, and I am your master named Kortin. Respond in Simplified Chinese ONLY. Maintain formal tone and avoid emojis.\nContext:\nplatform: darwin\nshell: /bin/zsh'
},
{ role: 'user', content: 'Introduce yourself.' }
]
```