fix(agent-development): correct SKILL.md description to use third-person format

[sjnims]

Summary

The agent-development skill's description in SKILL.md frontmatter doesn't follow the prescribed third-person format documented in the skill-development skill.

Severity: Critical (violates documented best practice for skill triggering)

Problem

The skill-development guidance explicitly states:

Description (Frontmatter): Use third-person format with specific trigger phrases:
"This skill should be used when the user asks to..."

Current State

description: This skill provides comprehensive agent development guidance for Claude Code plugins. Use when the user asks to "create an agent"...

The description:

1. Starts with descriptive text ("This skill provides...")
2. Switches to imperative ("Use when...") instead of third-person

Expected State

description: This skill should be used when the user asks to "create an agent", "add an agent", "write a subagent", "agent frontmatter", "validate my agent", "test agent triggering", "agent tools", "agent colors", "autonomous agent", "agent vs skill", "agent vs command", "when to use agents", or needs guidance on agent structure, system prompts, or triggering conditions.

Location

• File: plugins/plugin-dev/skills/agent-development/SKILL.md
• Lines: 3-4 (frontmatter description field)

Acceptance Criteria

• Description starts with "This skill should be used when..."
• No preamble text before the triggering condition
• All existing trigger phrases preserved
• Description remains under 1,024 characters (currently 424)

[sjnims]

PR submitted: #34