test(app/e2e): sessions-screen empty state (Cluster G7) (#742)

* test(app/e2e): cover sessions-screen empty state (Cluster G7)

Adds one E2E test — opens Sessions on a fresh workspace with no agents
and asserts the "No sessions yet" placeholder renders. Uses `withWorkspace`
fixture (no agent seeding) so the empty branch runs for the first time.

Also exports `expectSessionsEmptyState` helper from archive-tab helpers
for reuse in future session-related specs.

* fix(app/e2e): run sessions-empty test before archive-tab agents are created

The sessions screen shows global agent history for the daemon. Running
the empty-state test last meant the reconciliation tests had already
created 6 agents, so "No sessions yet" never rendered.

Moving the describe block first ensures it runs on a clean daemon
(workers:1, fullyParallel:false, archive-tab.spec.ts is first alphabetically).

* refactor(app/e2e): guard sessions-empty ordering + dedup selector constant

Addresses reviewer feedback on the ordering fragility:

- Add NOTE comment above Sessions screen empty state describe explaining
  why it must remain first in the file (daemon history is global; the
  reconciliation tests below call createIdleAgent which would pollute it).

- Add a fast-fail guard in expectSessionsEmptyState: asserts 0 agent rows
  with a 5s timeout so a future maintainer sees an immediately actionable
  failure message rather than a mysterious "No sessions yet" timeout.

- Extract AGENT_ROW_SELECTOR constant to eliminate the duplicated
  [data-testid^="agent-row-"] string shared by getSessionRowByTitle and
  the new guard.

* fix(app/e2e): move sessions-empty to 00-prefixed file to survive new specs

agent-stream-ui.spec.ts (merged in #743) sorts before archive-tab.spec.ts
and creates agents, breaking the empty-state test. Any future a*-*.spec.ts
has the same risk.

Fix: move the test to 00-sessions-empty.spec.ts — digit prefix sorts
before all alpha-named specs, making the ordering constraint explicit at
the filesystem level.

Also adds a beforeAll daemon probe that fails fast with a clear message
if any pre-existing agents are found, covering both ordering violations
and stale daemon state from a previous run.

Supporting changes:
- Add fetchAgentHistory to ArchiveTabDaemonClient interface (typed as
  Array<{ id: string }> — enough for the count check)
- Remove the test and NOTE comment from archive-tab.spec.ts
- Update expectSessionsEmptyState guard comment to reference the new file

packages/app/e2e/helpers/archive-tab.ts

@@ -36,6 +36,9 @@ interface ArchiveTabDaemonClient {
     predicate: (snapshot: { status: string }) => boolean,
     timeout?: number,
   ): Promise<{ status: string }>;
+  fetchAgentHistory(options?: {
+    page?: { limit: number };
+  }): Promise<{ entries: Array<{ id: string }> }>;
 }
 
 function getDaemonPort(): string {
@@ -265,8 +268,10 @@ export async function openSessions(page: Page): Promise<void> {
   });
 }
 
+const AGENT_ROW_SELECTOR = '[data-testid^="agent-row-"]';
+
 function getSessionRowByTitle(page: Page, title: string) {
-  return page.locator('[data-testid^="agent-row-"]').filter({ hasText: title }).first();
+  return page.locator(AGENT_ROW_SELECTOR).filter({ hasText: title }).first();
 }
 
 export async function expectSessionRowVisible(page: Page, title: string): Promise<void> {
@@ -283,6 +288,12 @@ export async function clickSessionRow(page: Page, title: string): Promise<void>
   await row.click();
 }
 
+export async function expectSessionsEmptyState(page: Page): Promise<void> {
+  // Guard: if session rows appear, a prior spec polluted the shared daemon — see 00-sessions-empty.spec.ts.
+  await expect(page.locator(AGENT_ROW_SELECTOR)).toHaveCount(0, { timeout: 5_000 });
+  await expect(page.getByText("No sessions yet")).toBeVisible({ timeout: 30_000 });
+}
+
 export async function archiveAgentFromSessions(
   page: Page,
   input: { agentId: string; title: string },