feat: strengthen agent-development skill trigger phrases

Replace fragment-style trigger phrases with natural query-style phrases
for better skill discovery and activation:
- "agent frontmatter" → "what are the agent frontmatter fields"
- "agent tools" → "how to restrict agent tools"
- "agent colors" → "what colors can agents use"
- Added: "design an agent for [task]"
- Added: "how do I write agent descriptions"
- Added: "validate my agent"
- Added: "test agent triggering"

Fixes #10

🤖 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", "agent frontmatter", "when to use description", "agent examples", "agent tools", "agent colors", "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", or needs guidance on agent structure, system prompts, triggering conditions, or agent development best practices for Claude Code plugins.
 version: 0.1.0
 ---