feat: agent variants — migration, API, service integration, frontend, tests

- Migration 027: agent_variants table with single-active enforcement,
  variant_id column on agent_performance_log
- API: full CRUD, clone from agent/variant, activate/deactivate,
  per-variant performance metrics and history endpoints
- Services: extractor, event classifier, thesis rewriter all wired
  to AgentConfigResolver with variant override support
- Frontend: variant list, comparison view, create/edit/clone forms,
  activate/delete actions on Agents page
- Tests: API tests + 5 property-based tests (single-active invariant,
  clone preservation, config resolution, slug determinism, update idempotence)
- Spec files for agent-variants feature

frontend/src/test/pages.test.tsx

@@ -168,3 +168,43 @@ describe('Global Events page', () => {
     });
   });
 });
+
+describe('Agents page', () => {
+  it('renders agent list in sidebar', async () => {
+    renderRoute('/agents');
+    await waitFor(() => {
+      expect(screen.getByText('Document Extractor')).toBeInTheDocument();
+      expect(screen.getByText('Event Classifier')).toBeInTheDocument();
+    });
+  });
+
+  it('renders variant list when an agent is selected', async () => {
+    renderRoute('/agents');
+    await waitFor(() => expect(screen.getByText('Document Extractor')).toBeInTheDocument());
+
+    await userEvent.click(screen.getByText('Document Extractor'));
+
+    await waitFor(() => {
+      expect(screen.getByText('GPT-4o Variant')).toBeInTheDocument();
+      expect(screen.getByText('Mistral Variant')).toBeInTheDocument();
+    });
+  });
+
+  it('shows comparison view when multiple variants are checked', async () => {
+    renderRoute('/agents');
+    await waitFor(() => expect(screen.getByText('Document Extractor')).toBeInTheDocument());
+
+    await userEvent.click(screen.getByText('Document Extractor'));
+
+    await waitFor(() => expect(screen.getByText('GPT-4o Variant')).toBeInTheDocument());
+
+    // Select both variant checkboxes
+    const checkboxes = screen.getAllByRole('checkbox');
+    await userEvent.click(checkboxes[0]);
+    await userEvent.click(checkboxes[1]);
+
+    await waitFor(() => {
+      expect(screen.getByText(/Variant Comparison/)).toBeInTheDocument();
+    });
+  });
+});