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.

uv.lock

@@ -465,7 +465,7 @@ wheels = [
 
 [[package]]
 name = "real-browser-cli"
-version = "0.15.3"
+version = "0.15.4"
 source = { editable = "." }
 dependencies = [
     { name = "click" },