feat: add ggshield linter for secret detection

[fsargent]

Summary

Adds GitGuardian CLI (ggshield) plugin for detecting hardcoded secrets in codebases.

Changes

• Added ggshield plugin configuration with standalone executable downloads
• Supports macOS (arm64, x86_64), Linux (x86_64), and Windows (x86_64)
• Includes custom SARIF parser for converting ggshield JSON output
• Adds test suite with snapshot validation
• Updates README.md to include ggshield in Security tools section
• Updates config check test snapshot to include ggshield

Features

• Scans all files for 450+ types of hardcoded secrets
• Uses standalone executables (no Python runtime required)
• Supports authentication via GITGUARDIAN_API_KEY environment variable
• Configurable via .gitguardian.yaml or .ggshield.yaml files
• Properly handles recursive directory scanning with strip_components

Testing

• ✅ Plugin configuration validates
• ✅ Test suite passes with snapshot
• ✅ Successfully detects secrets in test data
• ✅ Follows Trunk plugin best practices
• ✅ Config check test updated (all 224 repo tests passing)

Documentation

• Added README.md with setup and usage instructions
• Includes authentication setup guidance
• Documents configuration options