docs: add frontmatter to .github directory markdown files

- Add frontmatter to agent index
- Add frontmatter to prompt templates
- Update heading levels for consistency

Author: ashleyshaw

.github/agents/agent.md

@@ -1,3 +1,12 @@
+---
+title: Multi-Block Plugin Agents Index
+description: Main index of automation agents for multi-block plugin scaffold
+category: Agent
+type: Agent Index
+audience: Developers, AI Agents
+date: 2025-12-01
+---
+
 # Multi-Block Plugin Scaffold: Main Agent Index
 
 ## Overview & Related Files

.github/prompts/prompts.md

@@ -1,4 +1,13 @@
-# Multi-Block Plugin Scaffold: Prompt Templates & Authoring
+---
+title: Multi-Block Plugin Prompt Templates
+description: Prompt templates for consistent AI-assisted multi-block plugin development
+category: Documentation
+type: Prompt Templates
+audience: Developers, AI Assistants
+date: 2025-12-01
+---
+
+## Multi-Block Plugin Scaffold: Prompt Templates & Authoring
 
 ## Overview & Related Files