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>

tsconfig.e2e.json

@@ -0,0 +1,13 @@
+{
+  "extends": "./tsconfig.json",
+  "compilerOptions": {
+    "module": "CommonJS",
+    "moduleResolution": "Node",
+    "noEmit": false,
+    "outDir": "out",
+    "rootDir": ".",
+    "sourceMap": true,
+    "types": ["node", "mocha", "vscode"]
+  },
+  "include": ["src", "test/e2e"]
+}