feat: add environment filtering across all APIs and UI

Backend: Added optional `environment` query parameter to catalog,
search, stats, timeseries, punchcard, top-errors, logs, and agents
endpoints. ClickHouse queries filter by environment when specified
(literal SQL for AggregatingMergeTree, ? binds for raw tables).
StatsStore interface methods all accept environment parameter.

UI: Added EnvironmentSelector component (compact native select).
LayoutShell extracts distinct environments from agent data and
passes selected environment to catalog and agent queries via URL
search param (?env=). TopBar shows current environment label.

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

ui/src/components/EnvironmentSelector.tsx

@@ -0,0 +1,25 @@
+import styles from './EnvironmentSelector.module.css';
+
+interface EnvironmentSelectorProps {
+  environments: string[];
+  value: string | undefined;
+  onChange: (env: string | undefined) => void;
+}
+
+export function EnvironmentSelector({ environments, value, onChange }: EnvironmentSelectorProps) {
+  if (environments.length === 0) return null;
+
+  return (
+    <select
+      className={styles.select}
+      value={value ?? ''}
+      onChange={(e) => onChange(e.target.value || undefined)}
+      aria-label="Environment filter"
+    >
+      <option value="">All Envs</option>
+      {environments.map((env) => (
+        <option key={env} value={env}>{env}</option>
+      ))}
+    </select>
+  );
+}