feat: show distinct attribute keys in cmd-k Attributes tab

Add GET /search/attributes/keys endpoint that queries distinct
attribute key names from ClickHouse using JSONExtractKeys. Attribute
keys appear in the cmd-k Attributes tab alongside attribute value
matches from exchange results.

- SearchIndex.distinctAttributeKeys() interface method
- ClickHouseSearchIndex implementation using arrayJoin(JSONExtractKeys)
- SearchController /attributes/keys endpoint
- useAttributeKeys() React Query hook
- buildSearchData includes attribute keys as 'attribute' category items

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

ui/src/components/LayoutShell.tsx

@@ -3,7 +3,7 @@ import { AppShell, Sidebar, TopBar, CommandPalette, CommandPaletteProvider, Glob
 import type { SidebarApp, SearchResult } from '@cameleer/design-system';
 import { useRouteCatalog } from '../api/queries/catalog';
 import { useAgents } from '../api/queries/agents';
-import { useSearchExecutions } from '../api/queries/executions';
+import { useSearchExecutions, useAttributeKeys } from '../api/queries/executions';
 import { useAuthStore } from '../auth/auth-store';
 import { useState, useMemo, useCallback, useEffect, useRef } from 'react';
 import { ContentTabs } from './ContentTabs';
@@ -21,6 +21,7 @@ function healthToColor(health: string): string {
 function buildSearchData(
   catalog: any[] | undefined,
   agents: any[] | undefined,
+  attrKeys: string[] | undefined,
 ): SearchResult[] {
   if (!catalog) return [];
   const results: SearchResult[] = [];
@@ -61,6 +62,17 @@ function buildSearchData(
     }
   }
 
+  if (attrKeys) {
+    for (const key of attrKeys) {
+      results.push({
+        id: `attr-key-${key}`,
+        category: 'attribute',
+        title: key,
+        meta: 'attribute key',
+      });
+    }
+  }
+
   return results;
 }
 
@@ -94,6 +106,7 @@ function LayoutContent() {
   const { timeRange } = useGlobalFilters();
   const { data: catalog } = useRouteCatalog(timeRange.start.toISOString(), timeRange.end.toISOString());
   const { data: agents } = useAgents();
+  const { data: attributeKeys } = useAttributeKeys();
   const { username, logout } = useAuthStore();
   const { open: paletteOpen, setOpen: setPaletteOpen } = useCommandPalette();
   const { scope, setTab } = useScope();
@@ -141,8 +154,8 @@ function LayoutContent() {
   }, [catalog]);
 
   const catalogData = useMemo(
-    () => buildSearchData(catalog, agents as any[]),
-    [catalog, agents],
+    () => buildSearchData(catalog, agents as any[], attributeKeys),
+    [catalog, agents, attributeKeys],
   );
 
   // Stable reference for catalog data — only changes when catalog/agents actually change,