Fix plugin manifest version to match 3.4.0 release

Update .claude-plugin/plugin.json version from 3.2.3 to 3.4.0 to match
the version already set in marketplace.json. This ensures consistency
across all plugin manifest files.

.claude-plugin/plugin.json

@@ -1,7 +1,7 @@
 {
   "name": "superpowers",
   "description": "Core skills library for Claude Code: TDD, debugging, collaboration patterns, and proven techniques",
-  "version": "3.2.3",
+  "version": "3.4.0",
   "author": {
     "name": "Jesse Vincent",
     "email": "jesse@fsck.com"