#54: make undo-dependent E2E resilient to environments where undo is broken

`executeCommand("undo")` is non-functional in some headless `.vscode-test`
instances (it does not restore the buffer — see #54's diagnosis), which
false-fails every undo-dependent host E2E and turns `main`'s E2E red there even
though the product code is fine.

Add a RUNTIME preflight probe (`undoCapable.ts`): edit a scratch buffer, undo,
and report whether the buffer was actually restored (memoized per run). The #38
undoMarks suite gates on it via `suiteSetup` — running normally where undo works
(full coverage preserved, real regressions still caught) and skipping with a LOUD
console warning where it doesn't (no false red, no silent loss — the skip is
logged and shows as `pending`).

This fixes the "main E2E red" symptom. The underlying headless-undo limitation is
documented in #54; #40's undo-behavior coverage runs wherever undo works.

Test-infra only — no product code changed. Verified: with undo broken locally the
#38 suite skips (1 pending) and both E2E passes exit 0.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

test/e2e/suite/undoMarks.test.ts

@@ -3,6 +3,7 @@ import * as fs from "fs";
 import * as path from "path";
 import * as vscode from "vscode";
 import type { CowritingApi } from "../../../src/extension";
+import { undoWorks, UNDO_SKIP_REASON } from "./undoCapable";
 
 const WS = process.env.E2E_WORKSPACE!;
 const settle = () => new Promise((r) => setTimeout(r, 400));
@@ -34,6 +35,14 @@ suite("F10 #38 — undo does not mis-attribute restored text (host E2E, no LLM)"
   const DOC_REL = "docs/undo38.md";
   const BASE = "Alpha bravo charlie.\n";
 
+  // #54: skip (loudly) where executeCommand("undo") is non-functional; run where it works.
+  suiteSetup(async function () {
+    if (!(await undoWorks())) {
+      console.warn(UNDO_SKIP_REASON);
+      this.skip();
+    }
+  });
+
   test("undo of a deletion of baseline text leaves it unattributed (not human)", async () => {
     const { doc, key } = await freshDoc(DOC_REL, BASE);
     const api = await getApi();