docs: add CHANGELOG.md, release notes, and v2.1 polish

Add CHANGELOG.md (Keep a Changelog format) and release-notes.md
for the v2.1.0 GitHub Release. Also includes queued copy updates:
plugin.json version bump to 2.1.0, YouTube added to descriptions,
SKILL.md security/permissions section and ClawdBot metadata,
updated watchlist examples, and v2.1 launch post refinements.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.claude-plugin/marketplace.json

@@ -5,7 +5,8 @@
     "url": "https://github.com/mvanhorn"
   },
   "metadata": {
-    "description": "Research any topic from the last 30 days across Reddit, X, and the web"
+    "description": "Research any topic from the last 30 days across Reddit, X, YouTube, and the web",
+    "version": "2.1.0"
   },
   "plugins": [
     {