feat: Add Pre-commit Hooks guide

[bobbyonmagic]

Partial fix for #750 - adds Pre-commit Hooks guide (skill 8 of 24)

Changes

• 5 new guide files covering pre-commit framework, gitleaks, detect-secrets, and custom hooks
• Updated DevSecOps roadmap with link to guide

Guide Contents

1. Pre-commit Framework - Installation, configuration, CI integration, hook stages
2. Gitleaks - Secrets detection, custom rules, baseline files, GitHub Actions
3. Detect-secrets - Baseline-aware scanning, plugin architecture, audit workflow
4. Custom Security Hooks - Building organization-specific checks

Key Topics Covered

• Blocking sensitive file types (.pem, .key, .env)
• Detecting hardcoded secrets (AWS keys, tokens, passwords)
• Comparing gitleaks vs detect-secrets
• Writing custom hooks in Python, Bash, Node.js, Go
• Publishing shared hooks across repositories

All 4484 tests pass.

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