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/NewIssueDialog.tsx

@@ -94,6 +94,7 @@ export function NewIssueDialog() {
       setStatus(newIssueDefaults.status ?? "todo");
       setPriority(newIssueDefaults.priority ?? "");
       setProjectId(newIssueDefaults.projectId ?? "");
+      setAssigneeId(newIssueDefaults.assigneeAgentId ?? "");
     }
   }, [newIssueOpen, newIssueDefaults]);