feat: add proactive triggering example to agent-creator agent (#125)

## Summary

Added a 4th example to the `agent-creator` agent demonstrating proactive
triggering, where Claude recognizes implicit agent needs without
explicit "create an agent" requests.

## Problem

Fixes #121

The `agent-creator` agent had 3 examples all demonstrating reactive
triggering patterns. Per Claude Code subagent best practices, agents
should also demonstrate proactive triggering to show Claude when to
automatically invoke them.

## Solution

Added a new proactive triggering example that shows:
- User describing agent-like functionality without explicitly requesting
agent creation
- Assistant recognizing the implicit need and proactively triggering the
agent-creator

### Alternatives Considered

- Could add multiple proactive examples, but one well-crafted example
suffices
- Could modify existing examples to be proactive, but keeping reactive
examples is valuable for explicit requests

## Changes

- `plugins/plugin-dev/agents/agent-creator.md`: Added 4th example
demonstrating proactive triggering

## Testing

- [x] Agent validation script passes
- [x] Markdownlint passes
- [x] Example count now 4 (within recommended 2-4 range)

---
🤖 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

@@ -29,6 +29,15 @@ Plugin development with agent addition, trigger agent-creator.
 </commentary>
 </example>
 
+<example>
+Context: User describes needing autonomous functionality while discussing plugin development
+user: "My plugin needs something to automatically review code after I write it"
+assistant: "I'll use the agent-creator agent to generate a code review agent for your plugin."
+<commentary>
+User describes agent-like functionality need without explicitly requesting agent creation, proactively trigger agent-creator.
+</commentary>
+</example>
+
 # Explicit sonnet for complex agent generation reasoning
 model: sonnet
 color: magenta