Add hello-nebula and YouTube-dislike extensions

Add two Nebula extension packages:

- hello-nebula: a minimal demo extension that shows a small badge on pages (manifest, content.js, content.css, README).
- return-youtube-dislike: a content-script port that shows estimated YouTube dislike counts using the public Return YouTube Dislike API (manifest, content.js, content.css, README). Includes GPL-3.0 LICENSE and required attribution. READMEs contain install instructions and notes.

hello-nebula/content.js

@@ -0,0 +1,15 @@
+(function () {
+  if (document.getElementById('nebula-hello-badge')) {
+    return;
+  }
+
+  const badge = document.createElement('div');
+  badge.id = 'nebula-hello-badge';
+  badge.textContent = 'Nebula extension loaded';
+  badge.title = 'hello-nebula sample extension';
+  document.documentElement.appendChild(badge);
+
+  window.setTimeout(() => {
+    badge.classList.add('nebula-hello-badge--hide');
+  }, 3500);
+})();