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/manifest.json

@@ -0,0 +1,15 @@
+{
+  "id": "hello-nebula",
+  "name": "Hello Nebula",
+  "version": "1.0.0",
+  "description": "Sample Nebula extension. Shows a small badge on every page.",
+  "enabled": true,
+  "content_scripts": [
+    {
+      "matches": ["*://*/*"],
+      "js": ["content.js"],
+      "css": ["content.css"],
+      "run_at": "document_end"
+    }
+  ]
+}