feat: add cross-browser builds and refresh project documentation

- add Chrome/Chromium Manifest V3 build with PAC proxy routing
- keep Firefox Manifest V2 and Android compatibility
- add Chromium API adapter and service worker
- add Chrome build script and Makefile target
- update README and add AGENT.md
- document installation, signing, permissions, themes, and platform
  limitations

manifest.json

@@ -36,13 +36,10 @@
   "browser_specific_settings": {
     "gecko": {
       "id": "proxy-switcher-for-by-wt@9c0b2d3d",
-      "strict_min_version": "91.1.0",
-      "data_collection_permissions": {
-        "required": ["none"]
-      }
+      "strict_min_version": "91.1.0"
     },
     "gecko_android": {
-      "strict_min_version": "91.1.0"
+      "strict_min_version": "113.0"
     }
   }
 }