refactor: Reorganize repository structure for optimal learning path

Reorder folders based on learning dependencies, complexity, and frequency of use:
- 01-slash-commands (unchanged) - Quick wins for beginners
- 02-memory (was 03) - Essential foundation
- 03-skills (was 05) - Auto-invoked capabilities
- 04-subagents (was 02) - Task delegation
- 05-mcp (was 04) - External integration
- 06-hooks (was 07) - Event automation
- 07-plugins (was 06) - Bundled solutions
- 08-checkpoints (unchanged) - Safe experimentation
- 09-advanced-features (unchanged) - Power user tools

Documentation improvements:
- Add LEARNING-ROADMAP.md with detailed milestones and exercises
- Simplify README.md for better scannability
- Consolidate Quick Start and Getting Started sections
- Combine Feature Comparison and Use Case Matrix tables
- Reorder README sections: Learning Path → Quick Reference → Getting Started
- Update all cross-references across module READMEs

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

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

04-subagents/secure-reviewer.md

@@ -0,0 +1,18 @@
+---
+name: secure-reviewer
+description: Security-focused code review with minimal permissions
+tools: read, grep
+---
+
+# Secure Code Reviewer
+
+Reviews code for security vulnerabilities only.
+
+This agent:
+- ✅ Reads files to analyze
+- ✅ Searches for patterns
+- ❌ Cannot execute code
+- ❌ Cannot modify files
+- ❌ Cannot run tests
+
+This ensures the reviewer doesn't accidentally break anything.