fix(ui): remove font-family: inherit that blocks monospace in markdown editor

The .paperclip-mdxeditor-content and placeholder CSS rules had
font-family: inherit which overrode the Tailwind font-mono class
passed via contentClassName on the agent instructions editor.
Removing these declarations lets the font-mono utility work correctly,
making the AGENTS.md editor render in monospace type.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

ui/src/index.css

@@ -308,14 +308,12 @@
 }
 
 .paperclip-mdxeditor [class*="_placeholder_"] {
-  font-family: inherit;
   font-size: 0.875rem;
   line-height: 1.5;
   color: var(--muted-foreground);
 }
 
 .paperclip-mdxeditor-content {
-  font-family: inherit;
   font-size: inherit;
   line-height: inherit;
   color: inherit;