refactor(desktop): consolidate skills + tools management into one pane

The left-nav Skills pane and Settings > Skills & Tools rendered the same
getSkills()/getToolsets() data with the same helpers and toggles — genuine
duplication that drifted (different default category labels, sort orders).

Make the left pane the single home: it keeps its category-tabbed browsing
and now gains the functional bits it lacked — a real toolset enable/disable
switch (was a read-only pill) and the expandable ToolsetConfigPanel for
provider selection + per-key credential config. Remove the Tools section
from Settings (nav item, view branch, query slot, type union entries) and
delete tools-settings.tsx, migrating its toggle coverage into the skills
pane test. Relabel the entry point to 'Skills & Tools' in the sidebar and
command center.

apps/desktop/src/app/chat/sidebar/index.tsx

@@ -73,7 +73,7 @@ const SIDEBAR_NAV: SidebarNavItem[] = [
     icon: props => <Codicon name="robot" {...props} />,
     action: 'new-session'
   },
-  { id: 'skills', label: 'Skills', icon: props => <Codicon name="symbol-misc" {...props} />, route: SKILLS_ROUTE },
+  { id: 'skills', label: 'Skills & Tools', icon: props => <Codicon name="symbol-misc" {...props} />, route: SKILLS_ROUTE },
   { id: 'messaging', label: 'Messaging', icon: props => <Codicon name="comment" {...props} />, route: MESSAGING_ROUTE },
   { id: 'artifacts', label: 'Artifacts', icon: props => <Codicon name="files" {...props} />, route: ARTIFACTS_ROUTE }
 ]