fix: Change plugin name from 'auto-build' to 'ab' in marketplace.json

Fixes plugin discovery - the marketplace name must match what users install.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

.claude-plugin/marketplace.json

@@ -8,7 +8,7 @@
   },
   "plugins": [
     {
-      "name": "auto-build",
+      "name": "ab",
       "source": "./plugin",
       "description": "Spec-driven build orchestration with git worktree isolation and session memory",
       "version": "1.0.0",