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

scripts/build-chrome.py

@@ -0,0 +1,40 @@
+#!/usr/bin/env python3
+"""Build a Chrome/Chromium Manifest V3 zip without changing the Firefox tree."""
+from pathlib import Path
+import json
+import shutil
+import tempfile
+import zipfile
+
+ROOT = Path(__file__).resolve().parent.parent
+OUTPUT = ROOT / "web-ext-artifacts" / "proxyswitcherforkbywt-chrome.zip"
+FILES = [
+    "background-chrome.js",
+    "background-service-worker.js",
+    "browser-api.js",
+    "config-manager.js",
+    "options.css",
+    "options.html",
+    "options.js",
+    "popup.css",
+    "popup.html",
+    "popup.js",
+]
+
+def main():
+    OUTPUT.parent.mkdir(exist_ok=True)
+    with tempfile.TemporaryDirectory() as temp:
+        staging = Path(temp)
+        manifest = json.loads((ROOT / "manifest.chrome.json").read_text())
+        (staging / "manifest.json").write_text(json.dumps(manifest, indent=2) + "\n")
+        for name in FILES:
+            shutil.copy2(ROOT / name, staging / name)
+        shutil.copytree(ROOT / "icons", staging / "icons")
+        if OUTPUT.exists(): OUTPUT.unlink()
+        with zipfile.ZipFile(OUTPUT, "w", zipfile.ZIP_DEFLATED) as archive:
+            for path in sorted(staging.rglob("*")):
+                if path.is_file(): archive.write(path, path.relative_to(staging))
+    print(OUTPUT)
+
+if __name__ == "__main__":
+    main()