[2.29.0] Add schema-drift-detector agent

- New agent: schema-drift-detector for catching unrelated schema.rb changes in PRs
- Compares schema.rb diff against migrations in the PR
- Detects columns, indexes, tables from other branches
- Provides clear fix instructions
- Essential pre-merge check for database changes

Also updates component counts in README (were outdated)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.claude-plugin/marketplace.json

@@ -11,8 +11,8 @@
   "plugins": [
     {
       "name": "compound-engineering",
-      "description": "AI-powered development tools that get smarter with every use. Make each unit of engineering work easier than the last. Includes 28 specialized agents, 24 commands, and 15 skills.",
-      "version": "2.28.0",
+      "description": "AI-powered development tools that get smarter with every use. Make each unit of engineering work easier than the last. Includes 29 specialized agents, 24 commands, and 16 skills.",
+      "version": "2.29.0",
       "author": {
         "name": "Kieran Klaassen",
         "url": "https://github.com/kieranklaassen",