feat(agent-development): add comparative trigger phrases to skill description

Add 4 new trigger phrases for improved discoverability:
- "difference between agent and skill"
- "agent vs command"
- "when should I use an agent"
- "agent schema"

These help users discover the skill's comparison table when asking
conceptual questions about plugin components.

Fixes #17

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

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

Author: sjnims

plugins/plugin-dev/skills/agent-development/SKILL.md

@@ -1,6 +1,6 @@
 ---
 name: agent-development
-description: This skill should be used when the user asks to "create an agent", "add an agent", "write a subagent", "design an agent for [task]", "how do I write agent descriptions", "what are the agent frontmatter fields", "validate my agent", "test agent triggering", "how to restrict agent tools", "what colors can agents use", "autonomous agent", or needs guidance on agent structure, system prompts, triggering conditions, or agent development best practices for Claude Code plugins.
+description: This skill should be used when the user asks to "create an agent", "add an agent", "write a subagent", "design an agent for [task]", "how do I write agent descriptions", "what are the agent frontmatter fields", "validate my agent", "test agent triggering", "how to restrict agent tools", "what colors can agents use", "autonomous agent", "difference between agent and skill", "agent vs command", "when should I use an agent", "agent schema", or needs guidance on agent structure, system prompts, triggering conditions, or agent development best practices for Claude Code plugins.
 ---
 
 # Agent Development for Claude Code Plugins