Fix NCCoE blog post category: 'Standards Engagement' → 'Standards'

Build was failing because 'Standards Engagement' was not in the
mkdocs.yml categories_allowed list. Changed to 'Standards' which
is an existing allowed category.

Author: Claude-Ops

docs/blog/posts/2026-03-23-nccoe-response.md

@@ -2,7 +2,7 @@
 date: 2026-03-23
 slug: nccoe-ai-agent-identity-authorization-response
 categories:
-  - Standards Engagement
+  - Standards
 authors:
   - nikhil
 description: "Response to the NCCoE concept paper on AI agent identity and authorization. The paper proposes intent conveyance as the missing layer: agents authenticate and get authorized, but nothing in the identity stack evaluates whether their actions match the purpose they were authorized for."