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/pages/Issues.tsx

@@ -89,22 +89,23 @@ export function Issues() {
   return (
     <div className="space-y-4">
       <div className="flex items-center justify-between">
-        <h2 className="text-lg font-semibold">Issues</h2>
+        <div className="flex items-center gap-4">
+          <h2 className="text-lg font-semibold">Issues</h2>
+          <Tabs value={tab} onValueChange={(v) => setTab(v as TabFilter)}>
+            <TabsList>
+              <TabsTrigger value="all">All Issues</TabsTrigger>
+              <TabsTrigger value="active">Active</TabsTrigger>
+              <TabsTrigger value="backlog">Backlog</TabsTrigger>
+              <TabsTrigger value="done">Done</TabsTrigger>
+            </TabsList>
+          </Tabs>
+        </div>
         <Button size="sm" onClick={() => openNewIssue()}>
           <Plus className="h-4 w-4 mr-1" />
           New Issue
         </Button>
       </div>
 
-      <Tabs value={tab} onValueChange={(v) => setTab(v as TabFilter)}>
-        <TabsList>
-          <TabsTrigger value="all">All Issues</TabsTrigger>
-          <TabsTrigger value="active">Active</TabsTrigger>
-          <TabsTrigger value="backlog">Backlog</TabsTrigger>
-          <TabsTrigger value="done">Done</TabsTrigger>
-        </TabsList>
-      </Tabs>
-
       {isLoading && <p className="text-sm text-muted-foreground">Loading...</p>}
       {error && <p className="text-sm text-destructive">{error.message}</p>}