feat: add agent lineage visualization with SVG tree rendering

Add a dedicated /lineage route with custom SVG-based tree visualization
of the agent spawn chain. Nodes are colored by agent type (orchestrator,
researcher, coder, sysadmin, assistant) and display agent ID, type, and
spawn depth. Clicking a node opens a detail panel. Uses sample data
since the API does not yet expose lineage information.

Closes #15

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/routes/chat/+page.svelte

@@ -29,6 +29,7 @@
 		create(SessionConfigSchema, { overrideLevel: OverrideLevel.NONE })
 	);
 	let showConfig = $state(false);
+	const lineageHref = resolveRoute('/lineage');
 
 	const isNonDefaultConfig = $derived(
 		sessionConfig.overrideLevel !== OverrideLevel.NONE ||
@@ -136,6 +137,15 @@
 	<div class="flex flex-1 flex-col">
 		<header class="flex items-center justify-between border-b border-gray-200 bg-white px-4 py-3">
 			<h1 class="text-lg font-semibold text-gray-900">Chat</h1>
+			<div class="flex items-center gap-2">
+			<!-- eslint-disable svelte/no-navigation-without-resolve -- resolveRoute is resolve; plugin does not recognize the alias -->
+			<a
+				href={lineageHref}
+				class="rounded-lg px-2.5 py-1.5 text-sm text-gray-600 hover:bg-gray-100 hover:text-gray-900"
+			>
+				Lineage
+			</a>
+			<!-- eslint-enable svelte/no-navigation-without-resolve -->
 			<button
 				type="button"
 				onclick={() => (showConfig = !showConfig)}
@@ -147,6 +157,7 @@
 				{/if}
 				Config
 			</button>
+			</div>
 		</header>
 
 		<MessageList {messages} />