Add OpenScript.button popup actions and release 1.0.6

Scripts can register buttons at runtime with OpenScript.button(label, onClick).
The popup lists buttons from scripts running in the active tab via
chrome.userScripts.execute in each script's world, and clicking one runs
the handler in the page before closing the popup.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

manifest.json

@@ -2,7 +2,7 @@
   "manifest_version": 3,
   "minimum_chrome_version": "138",
   "name": "OpenScript",
-  "version": "1.0.5",
+  "version": "1.0.6",
   "description": "A lightweight user script manager for modern browsers",
   "action": {
     "default_popup": "src/popup.html",