fix(install): include agents folder in npm package

The agents folder (gsd-executor, gsd-verifier, gsd-integration-checker,
gsd-milestone-auditor) was not being published to npm because it was
missing from the package.json files array.

Also adds the missing gsd-milestone-auditor.md agent.

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

package.json

@@ -8,7 +8,8 @@
   "files": [
     "bin",
     "commands",
-    "get-shit-done"
+    "get-shit-done",
+    "agents"
   ],
   "keywords": [
     "claude",