test(ui): await initial roster before digest assertions

The final digest rendered correctly, but the test captured zero matching
sessions.list requests before initial roster bootstrap was released. Its
no-extra-read assertion then mistook the first roster read for an event
reload (expected length 0, received 1).

Introducing ordering: b023df8086 (#146591) prioritizes selected chat
history over automatic background lists. A selected descriptor can already
render the sidebar row while chat.startup still holds that first list.

Hold chat.startup to exercise the early descriptor path, release it, then
wait for a second row available only from roster admission before recording
the request baseline. Preserve the exact final-digest and no-extra-list-read
assertions. No production change, retry, timeout increase, or sleep is added.

Probe evidence on main 1a94ee9ed7:
- Original whole file: 5 passed, target failed with the CI 0-to-1 signature.
- Holding startup until after the final digest deterministically reproduces
  the same failure; the final digest is visible before the assertion fails.
- The identical probe passes with only the background-startup gate introduced
  by b023df8086 temporarily removed. That production change was reverted.
- Suspect-range samples before and at b7e2666e6b and at 3a306a430e pass the
  unmodified file. Snapshot stamping is not the stale-digest cause: static
  list fixtures already seed canonical descriptors, and the digest renders.
- Full replay at b023df8086 cannot build with the installed dependency set
  (missing historical ms dependency); no dependencies were installed. The
  attribution uses its parent-relative patch plus the isolated gate control.

Validation:
- Complete sidebar presentation E2E file: 6 passed, 49.08s wall including
  bundle build; changed scenario 1.405s.
- Bootstrap, sidebar narration, roster event and reconciliation owners:
  4 files / 107 tests passed, one worker, 29.23s runner wall.
- pnpm tsgo:ui, oxfmt, targeted core-tsconfig oxlint, git diff --check.
- Line-cap, max-lines suppression and assertion-safety ratchets pass.
  The combined max-lines command also reports the existing environment-name
  budget failure (484 vs 483), reproduced from the untouched HEAD index.
- Independent Codex autoreview: scoped-clean through P2.

Release-note context: test reliability only; product behavior is unchanged.

ui/src/e2e/chat-flow.sidebar-presentation.e2e.test.ts

@@ -162,6 +162,12 @@ suite.define(() => {
     });
     const key = "agent:main:session-a";
     const runId = "run-sidebar-metadata";
+    const rosterPeer = {
+      key: "agent:main:roster-peer",
+      kind: "direct",
+      label: "Roster peer",
+      updatedAt: 1,
+    };
     const running = chatSessionListResponse([
       {
         key,
@@ -180,6 +186,7 @@ suite.define(() => {
           revision: 1,
         },
       },
+      rosterPeer,
     ]);
     const completed = chatSessionListResponse([
       {
@@ -200,8 +207,10 @@ suite.define(() => {
           revision: 2,
         },
       },
+      rosterPeer,
     ]);
     const gateway = await installMockGateway(page, {
+      deferredMethods: ["chat.startup"],
       methodResponses: { "sessions.list": running },
       sessionKey: key,
     });
@@ -210,6 +219,11 @@ suite.define(() => {
       await page.goto(controlUiSessionUrl(suite.server.baseUrl, key));
       const row = page.locator(`.sidebar-recent-session[data-session-key="${key}"]`);
       await row.getByText("Implementing the repair").waitFor();
+      // A descriptor can render the selected row before startup releases the roster.
+      // Wait for a roster-only row before measuring event-triggered list reads.
+      await gateway.waitForRequest("chat.startup");
+      await gateway.resolveDeferred("chat.startup");
+      await page.locator(`.sidebar-recent-session[data-session-key="${rosterPeer.key}"]`).waitFor();
       if (captureUiProofEnabled) {
         await writeFile(
           path.join(