feat(ui): M4 — agent activity, knowledge search, audit, correlation grouping

Add three new pages completing the control-room web UI:
- Agent activity: polls /agent-activity every 5s, filterable by type/agent
- Knowledge search: FTS over /knowledge/search with snippet + entity links
- Audit trail: browseable audit log with actor/action/entity filters

Enhanced live events page with correlation-id clustering (Groups toggle).
Added fetchAgentActivity/searchKnowledge/fetchAudit to the API client.
11 nav items now cover all planned control-room views.

web/src/lib/components/ui/select/select-group-heading.svelte

@@ -0,0 +1,21 @@
+<script lang="ts">
+	import { Select as SelectPrimitive } from "bits-ui";
+	import { cn } from "$lib/utils.js";
+	import type { ComponentProps } from "svelte";
+
+	let {
+		ref = $bindable(null),
+		class: className,
+		children,
+		...restProps
+	}: ComponentProps<typeof SelectPrimitive.GroupHeading> = $props();
+</script>
+
+<SelectPrimitive.GroupHeading
+	bind:ref
+	data-slot="select-group-heading"
+	class={cn("text-muted-foreground px-2 py-1.5 text-xs", className)}
+	{...restProps}
+>
+	{@render children?.()}
+</SelectPrimitive.GroupHeading>