refactor(vscode): use consistent kilo-ui styling in agent manager (#473)

Replace hardcoded values with kilo-ui design tokens throughout the
agent manager CSS: border-radius uses --radius-* tokens, font sizes
use --font-size-* tokens, colors use single semantic token references
instead of triple-fallback var() chains with hex defaults, and
interactive states use :active opacity instead of CSS transitions.
Replace raw tab close <button> with IconButton component.