Allow independent font size setting for agent panel

[gh0stintheshe11]

Body

The agent chat panel currently inherits its font size from ui_font_size, which controls all UI elements (tabs, file explorer, menus, etc.).

I'd like a dedicated setting like agent_font_size (or panel_font_size) to control the agent chat response text independently.

Why?

• Agent responses contain dense text that benefits from a larger font
• But increasing ui_font_size makes everything else (tabs, sidebars, menus) unnecessarily large
• The code blocks in agent responses already render at a comfortable size, but the body text is noticeably smaller

Suggested Setting

{
  "agent": {
    "font_size": 18
  }
}

[SomeoneToIgnore]

https://github.com/zed-industries/zed/discussions?discussions_q=is%3Aopen+agent+font