feat: brain sub-sidebar in AppShell — folders/tags under Brain nav item

- Brain nav item expands when on /brain page
- Shows Folders/Tags toggle tabs
- Folder links: /brain?folder=Work etc
- Tag links: /brain?tag=dev etc
- Counts shown next to each
- Brain page reads filter from URL params
- Only shows folders with items + key defaults (Home, Work, Knowledge)
- Tags limited to 12 most-used

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

frontend-v2/src/lib/components/layout/AppShell.svelte

@@ -1,5 +1,6 @@
 <script lang="ts">
 	import { page } from '$app/state';
+	import { onMount } from 'svelte';
 	import {
 		BookOpen,
 		Brain,
@@ -7,6 +8,7 @@
 		CircleDot,
 		Compass,
 		Dumbbell,
+		FolderOpen,
 		Landmark,
 		LibraryBig,
 		Menu,
@@ -14,6 +16,7 @@
 		Search,
 		Settings2,
 		SquareCheckBig,
+		Tag,
 		X
 	} from '@lucide/svelte';
 
@@ -50,6 +53,33 @@
 
 	let mobileNavOpen = $state(false);
 
+	// Brain sidebar sub-items
+	interface BrainFolder { id: string; name: string; item_count: number; }
+	interface BrainTag { id: string; name: string; item_count: number; }
+	let brainFolders = $state<BrainFolder[]>([]);
+	let brainTags = $state<BrainTag[]>([]);
+	let brainSubView = $state<'folders' | 'tags'>('folders');
+	let brainExpanded = $derived(page.url.pathname.startsWith('/brain'));
+
+	async function loadBrainSidebar() {
+		try {
+			const res = await fetch('/api/brain/taxonomy/sidebar', { credentials: 'include' });
+			if (res.ok) {
+				const data = await res.json();
+				brainFolders = data.folders || [];
+				brainTags = data.tags || [];
+			}
+		} catch { /* silent */ }
+	}
+
+	onMount(() => {
+		if (brainExpanded) loadBrainSidebar();
+	});
+
+	$effect(() => {
+		if (brainExpanded) loadBrainSidebar();
+	});
+
 	function isActive(href: string): boolean {
 		if (href === '/') return page.url.pathname === '/';
 		return page.url.pathname === href || page.url.pathname.startsWith(href + '/');
@@ -82,6 +112,36 @@
 						<item.icon size={16} strokeWidth={1.8} />
 						<span>{item.label}</span>
 					</a>
+					{#if item.id === 'brain' && brainExpanded}
+						<div class="rail-sub">
+							<div class="rail-sub-tabs">
+								<button class="rail-sub-tab" class:active={brainSubView === 'folders'} onclick={() => brainSubView = 'folders'}>
+									<FolderOpen size={11} strokeWidth={2} /> Folders
+								</button>
+								<button class="rail-sub-tab" class:active={brainSubView === 'tags'} onclick={() => brainSubView = 'tags'}>
+									<Tag size={11} strokeWidth={2} /> Tags
+								</button>
+							</div>
+							{#if brainSubView === 'folders'}
+								<a href="/brain" class="rail-sub-item" class:active={page.url.pathname === '/brain' && !page.url.searchParams.get('folder')}>
+									<span>All</span>
+								</a>
+								{#each brainFolders.filter(f => f.item_count > 0 || f.name === 'Home' || f.name === 'Work' || f.name === 'Knowledge') as folder}
+									<a href="/brain?folder={folder.name}" class="rail-sub-item" class:active={page.url.searchParams.get('folder') === folder.name}>
+										<span>{folder.name}</span>
+										<span class="rail-sub-count">{folder.item_count}</span>
+									</a>
+								{/each}
+							{:else}
+								{#each brainTags.filter(t => t.item_count > 0).slice(0, 12) as tag}
+									<a href="/brain?tag={tag.name}" class="rail-sub-item" class:active={page.url.searchParams.get('tag') === tag.name}>
+										<span>{tag.name}</span>
+										<span class="rail-sub-count">{tag.item_count}</span>
+									</a>
+								{/each}
+							{/if}
+						</div>
+					{/if}
 				{/each}
 			</nav>
 		</div>
@@ -280,6 +340,64 @@
 		transform: translateX(3px);
 	}
 
+	/* Brain sub-sidebar */
+	.rail-sub {
+		padding: 4px 0 4px 18px;
+		display: grid;
+		gap: 2px;
+	}
+	.rail-sub-tabs {
+		display: flex;
+		gap: 2px;
+		margin-bottom: 4px;
+		padding: 0 6px;
+	}
+	.rail-sub-tab {
+		display: flex;
+		align-items: center;
+		gap: 4px;
+		padding: 4px 8px;
+		border-radius: 6px;
+		border: none;
+		background: none;
+		font-size: 0.7rem;
+		font-weight: 600;
+		color: var(--shell-muted);
+		font-family: inherit;
+		transition: all 140ms;
+	}
+	.rail-sub-tab.active {
+		background: rgba(255,255,255,0.5);
+		color: var(--shell-ink);
+	}
+	.rail-sub-item {
+		display: flex;
+		align-items: center;
+		justify-content: space-between;
+		gap: 6px;
+		padding: 6px 10px;
+		border-radius: 8px;
+		color: var(--shell-muted);
+		font-size: 0.78rem;
+		transition: all 140ms;
+		text-decoration: none;
+	}
+	.rail-sub-item:hover {
+		background: rgba(255,255,255,0.4);
+		color: var(--shell-ink);
+	}
+	.rail-sub-item.active {
+		background: rgba(255,255,255,0.6);
+		color: var(--shell-ink);
+		font-weight: 600;
+	}
+	.rail-sub-count {
+		font-size: 0.65rem;
+		font-family: var(--mono, monospace);
+		color: var(--shell-muted);
+		opacity: 0.7;
+	}
+
 	.rail-bottom {
 		display: grid;
 		gap: 10px;

frontend-v2/src/lib/pages/brain/AtelierBrainPage.svelte

@@ -307,6 +307,16 @@
   });
 
   onMount(async () => {
+    // Read filters from URL params (set by AppShell sidebar links)
+    const urlFolder = new URL(window.location.href).searchParams.get('folder');
+    const urlTag = new URL(window.location.href).searchParams.get('tag');
+    if (urlFolder) {
+      activeFolder = urlFolder;
+    }
+    if (urlTag) {
+      activeTag = urlTag;
+    }
+
     await loadSidebar();
     await loadItems();
   });