feat: establish root config and workspace foundation

- Create browser-ext placeholder package.json for workspace resolution
- Update root engines to node >=22 matching .nvmrc and web/package.json
- pnpm-workspace.yaml already configured with web and browser-ext
- All legacy directories (packages/, services/, server/) already removed

browser-ext/package.json

@@ -0,0 +1,10 @@
+{
+  "name": "browser-ext",
+  "version": "0.1.0",
+  "private": true,
+  "scripts": {
+    "build": "echo 'browser-ext build placeholder'",
+    "dev": "echo 'browser-ext dev placeholder'",
+    "test": "echo 'browser-ext test placeholder'"
+  }
+}