Put system prompt configuration at ai level

[pxp9]

Currently conf is like this

{
  "ai": {
    "provider": "google_ai",
    "google_ai": {
      "api_key": "your_key",
      "model": "gemini-2.5-flash-preview",
      "system_prompt": "Your custom system prompt here"
    }
  },
  "datetime_format": "%H:%M",
  "keybindings": {
    "quit": "q",
    "quit_ctrl": "ctrl+c",
    "switch_mode": "tab",
    "navigate_up": "k",
    "navigate_up_alt": "up",
    "navigate_down": "j",
    "navigate_down_alt": "down",
    "select": "enter",
    "backspace": "backspace",
    "reload_config": "ctrl+r"
  }
}

we should put the system prompt in the provider level

{
  "ai": {
    "provider": "google_ai",
    "system_prompt": "Your custom system prompt here",
    "google_ai": {
      "api_key": "your_key",
      "model": "gemini-2.5-flash-preview"
    }
  },
  "datetime_format": "%H:%M",
  "keybindings": {
    "quit": "q",
    "quit_ctrl": "ctrl+c",
    "switch_mode": "tab",
    "navigate_up": "k",
    "navigate_up_alt": "up",
    "navigate_down": "j",
    "navigate_down_alt": "down",
    "select": "enter",
    "backspace": "backspace",
    "reload_config": "ctrl+r"
  }
}

this way this option is reusable per provider.