feat(release): v2.9.1 - auto-save research to ~/Documents/Last30Days/

Bump version to 2.9.1, update changelog and release notes.
Credit @devin_explores for inspiring the feature.

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

.claude-plugin/plugin.json

@@ -1,7 +1,7 @@
 {
   "name": "last30days",
   "description": "Research any topic from the last 30 days across Reddit, X, YouTube, and the web",
-  "version": "2.1.0",
+  "version": "2.9.1",
   "author": {
     "name": "mvanhorn"
   },