chore: prepare verified CRX uploads and release 0.15.4

- Add helper scripts for Chrome Web Store verified CRX uploads using a dedicated RSA upload key protected by GPG.
- Document the verified upload packaging flow and ignore local signing secrets.
- Add npm packaging entry point for signed webstore CRX artifacts.
- Chunk large SDK tab close batches to avoid native-host response timeouts.
- Bump project and extension versions to 0.15.4 with matching tests.

extension/manifest.json

@@ -1,7 +1,7 @@
 {
   "manifest_version": 3,
   "name": "browser-cli",
-  "version": "0.15.3",
+  "version": "0.15.4",
   "description": "Control your browser from the terminal or Python SDK",
   "browser_specific_settings": {
     "gecko": {