test+docs(workbench-seeding): hook stamping test + per-space-seeds guide

Closes the two remaining gaps after the seeding-cleanup landed:

- `data/space-stamping.test.ts` exercises the smart-hook contract
  end-to-end against fake-indexeddb. Four scenarios: active Brand
  Space → row carries Brand UUID; no active Space → personal sentinel;
  explicit spaceId on the record is preserved verbatim; flipping the
  active Space between writes flips the stamp. The Brand-Space case
  is the regression guard for the original bug (writes silently
  routing to Personal after `reconcileSentinels`).
- `apps/mana/CLAUDE.md` gets a Per-Space Seeds section so the next
  module dev who needs to pre-populate something on Space activation
  finds the `registerSpaceSeed` pattern + `data/seeds/index.ts` barrel
  + the deterministic-id discipline without grepping the codebase.
  Reference impl link points at workbench-home.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

apps/mana/apps/web/src/lib/data/space-stamping.test.ts

@@ -0,0 +1,123 @@
+/**
+ * Tests for the Dexie creating-hook's tenancy stamping. The hook in
+ * `database.ts` calls `getEffectiveSpaceId()` on every write to a
+ * space-scoped table that didn't pre-set `spaceId`, so:
+ *
+ *   - With an active Space loaded, writes land under that Space's UUID.
+ *   - During the bootstrap window (no active Space), writes carry the
+ *     personal sentinel `_personal:<userId>`, which `reconcileSentinels`
+ *     rewrites once `loadActiveSpace` resolves the real id.
+ *   - An explicitly-set spaceId on the record is preserved verbatim.
+ *
+ * Before the smart hook landed, the literal `_personal:<userId>` was
+ * stamped unconditionally — so writes during a Brand-Space session
+ * silently routed to Personal after `reconcileSentinels` rewrote them.
+ * The tests here are the regression guard for that bug.
+ */
+
+import 'fake-indexeddb/auto';
+import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest';
+
+vi.mock('$lib/stores/funnel-tracking', () => ({ trackFirstContent: vi.fn() }));
+vi.mock('$lib/triggers/registry', () => ({ fire: vi.fn() }));
+vi.mock('$lib/triggers/inline-suggest', () => ({
+	checkInlineSuggestion: vi.fn().mockResolvedValue(null),
+}));
+
+import { db } from './database';
+import { setCurrentUserId } from './current-user';
+import { setActiveSpace } from './scope/active-space.svelte';
+import type { ActiveSpace } from './scope/active-space.svelte';
+
+const brandSpace: ActiveSpace = {
+	id: 'space-brand-uuid',
+	slug: 'acme',
+	name: 'Acme Brand',
+	type: 'brand',
+	tier: 'public',
+	role: 'owner',
+};
+
+beforeEach(async () => {
+	setCurrentUserId('test-user');
+	setActiveSpace(null);
+	await db.table('tasks').clear();
+});
+
+afterEach(() => {
+	setActiveSpace(null);
+	setCurrentUserId(null);
+});
+
+describe('creating-hook tenancy stamping', () => {
+	it('stamps the active Space id when one is loaded', async () => {
+		setActiveSpace(brandSpace);
+
+		await db.table('tasks').add({
+			id: 'task-stamp-1',
+			title: 'in brand',
+			priority: 'medium',
+			isCompleted: false,
+			order: 0,
+		});
+
+		const row = await db.table('tasks').get('task-stamp-1');
+		expect(row?.spaceId).toBe('space-brand-uuid');
+	});
+
+	it('falls back to the personal sentinel when no Space is loaded', async () => {
+		await db.table('tasks').add({
+			id: 'task-stamp-2',
+			title: 'no space yet',
+			priority: 'low',
+			isCompleted: false,
+			order: 0,
+		});
+
+		const row = await db.table('tasks').get('task-stamp-2');
+		expect(row?.spaceId).toBe('_personal:test-user');
+	});
+
+	it('preserves an explicitly-set spaceId verbatim', async () => {
+		// Cross-space write pattern (e.g. workbench-home seeder
+		// writing into a target Space that isn't the active one).
+		setActiveSpace(brandSpace);
+
+		await db.table('tasks').add({
+			id: 'task-stamp-3',
+			title: 'forced personal',
+			priority: 'medium',
+			isCompleted: false,
+			order: 0,
+			spaceId: 'space-explicit-target',
+		});
+
+		const row = await db.table('tasks').get('task-stamp-3');
+		expect(row?.spaceId).toBe('space-explicit-target');
+	});
+
+	it('updates the stamp when the active Space changes between writes', async () => {
+		setActiveSpace(brandSpace);
+		await db.table('tasks').add({
+			id: 'task-stamp-4a',
+			title: 'first',
+			priority: 'low',
+			isCompleted: false,
+			order: 0,
+		});
+
+		setActiveSpace(null);
+		await db.table('tasks').add({
+			id: 'task-stamp-4b',
+			title: 'second',
+			priority: 'low',
+			isCompleted: false,
+			order: 1,
+		});
+
+		const a = await db.table('tasks').get('task-stamp-4a');
+		const b = await db.table('tasks').get('task-stamp-4b');
+		expect(a?.spaceId).toBe('space-brand-uuid');
+		expect(b?.spaceId).toBe('_personal:test-user');
+	});
+});