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

Makefile

@@ -3,5 +3,8 @@ default: build
 build:
 	web-ext build --overwrite-dest --filename proxyswitcherforkbywt.xpi
 
+build-chrome:
+	python scripts/build-chrome.py
+
 clean:
 	rm -rvf web-ext-artifacts