F2 SLICE-5: @vscode/test-electron host E2E (create→reload→re-anchor→orphan) (#4)

Drives the extension's returned ThreadController API + asserts the on-disk
sidecar and rendered Comments state (spec §6.8 fallback). All 4 host tests
green against VS Code 1.124.0.

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

test/e2e/runTest.ts

@@ -0,0 +1,31 @@
+import * as path from "path";
+import * as os from "os";
+import * as fs from "fs";
+import { runTests } from "@vscode/test-electron";
+
+async function main(): Promise<void> {
+  const projectRoot = path.resolve(__dirname, "../../../");
+  const extensionDevelopmentPath = projectRoot;
+  const extensionTestsPath = path.resolve(__dirname, "./suite/index");
+
+  // Copy the fixture workspace to a temp dir so test writes are throwaway.
+  const fixture = path.resolve(projectRoot, "test/e2e/fixtures/workspace");
+  const workspace = fs.mkdtempSync(path.join(os.tmpdir(), "cowriting-e2e-"));
+  fs.cpSync(fixture, workspace, { recursive: true });
+
+  try {
+    await runTests({
+      extensionDevelopmentPath,
+      extensionTestsPath,
+      launchArgs: [workspace, "--disable-extensions"],
+      extensionTestsEnv: { E2E_WORKSPACE: workspace },
+    });
+  } finally {
+    fs.rmSync(workspace, { recursive: true, force: true });
+  }
+}
+
+main().catch((err) => {
+  console.error("E2E run failed:", err);
+  process.exit(1);
+});