feat: add skills field to agents for auto-loading relevant skills

Add the `skills` frontmatter field to each agent to auto-load relevant
skill documentation when the agent is invoked:

- agent-creator: loads agent-development skill
- plugin-validator: loads plugin-structure, hook-development,
  command-development, skill-development, agent-development skills
- skill-reviewer: loads skill-development skill

This makes agents more self-contained by automatically providing them
with the plugin's best practices documentation for their domain.

Fixes #118

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

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

Author: sjnims

plugins/plugin-dev/agents/agent-creator.md

@@ -33,6 +33,7 @@ Plugin development with agent addition, trigger agent-creator.
 model: sonnet
 color: magenta
 tools: Write, Read
+skills: agent-development
 ---
 
 You are an elite AI agent architect specializing in crafting high-performance agent configurations. Your expertise lies in translating user requirements into precisely-tuned agent specifications that maximize effectiveness and reliability.

plugins/plugin-dev/agents/plugin-validator.md

@@ -41,6 +41,7 @@ Marketplace created, validate schema and plugin entries.
 model: inherit
 color: yellow
 tools: Read, Grep, Glob, Bash
+skills: plugin-structure, hook-development, command-development, skill-development, agent-development
 ---
 
 You are an expert plugin and marketplace validator specializing in comprehensive validation of Claude Code plugin structure, configuration, components, and plugin marketplaces.

plugins/plugin-dev/agents/skill-reviewer.md

@@ -32,6 +32,7 @@ Skill description modified, review for triggering effectiveness.
 model: inherit
 color: cyan
 tools: Read, Grep, Glob
+skills: skill-development
 ---
 
 You are an expert skill architect specializing in reviewing and improving Claude Code skills for maximum effectiveness and reliability.