UI polish: sidebar nav, Companies/Issues layout, assignee in issue defaults

- Sidebar: add Approvals and Companies links, remove temporary Design Guide link
- Companies: replace inline create form with 'New Company' button (opens onboarding)
- Issues: move filter tabs inline with page heading (consistent with Agents)
- DialogContext: add assigneeAgentId to NewIssueDefaults
- NewIssueDialog: wire assigneeAgentId default (pre-select assignee when opening
  from agent detail page)
- CommandPalette: minor cleanup
- App: add DesignGuide route (dev-only, no sidebar link)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

ui/src/components/CommandPalette.tsx

@@ -25,7 +25,6 @@ import {
   Inbox,
   DollarSign,
   History,
-  GitBranch,
   SquarePen,
   Plus,
 } from "lucide-react";
@@ -114,10 +113,6 @@ export function CommandPalette() {
             <History className="mr-2 h-4 w-4" />
             Activity
           </CommandItem>
-          <CommandItem onSelect={() => go("/org")}>
-            <GitBranch className="mr-2 h-4 w-4" />
-            Org Chart
-          </CommandItem>
         </CommandGroup>
 
         <CommandSeparator />