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

browser-api.js

@@ -0,0 +1,31 @@
+// Small Promise-based WebExtension API adapter for Chromium browsers.
+// Firefox already provides the browser namespace, so this is a no-op there.
+(function () {
+  if (globalThis.browser || !globalThis.chrome) return;
+
+  const wrap = (object, method) => (...args) => new Promise((resolve, reject) => {
+    object[method](...args, result => {
+      const error = globalThis.chrome.runtime.lastError;
+      if (error) reject(new Error(error.message));
+      else resolve(result);
+    });
+  });
+
+  globalThis.browser = {
+    runtime: {
+      sendMessage: wrap(chrome.runtime, 'sendMessage'),
+      openOptionsPage: wrap(chrome.runtime, 'openOptionsPage'),
+      onMessage: chrome.runtime.onMessage
+    },
+    tabs: {
+      query: wrap(chrome.tabs, 'query'),
+      reload: wrap(chrome.tabs, 'reload')
+    },
+    storage: {
+      local: {
+        get: wrap(chrome.storage.local, 'get'),
+        set: wrap(chrome.storage.local, 'set')
+      }
+    }
+  };
+})();