This commit addresses security concerns identified during code review
by establishing proper security policies and automated monitoring.

## New Security Infrastructure

### SECURITY.md
- Establishes vulnerability reporting process
- Documents security best practices for users
- Outlines supported versions and disclosure policy
- Provides security hardening recommendations
- Lists security considerations for development

### Dependabot Configuration
- Automated weekly dependency updates
- Separate configs for Cargo and GitHub Actions
- Grouped minor/patch updates for easier review
- Automatic labeling and assignment
- Reduces time to address vulnerabilities

### Security Maintenance Guide
- Step-by-step instructions for addressing vulnerabilities
- Commands for cargo audit, update, and outdated
- Detailed vulnerability resolution workflow
- Testing procedures after updates
- Rollback strategy for problematic updates
- Prevention and monitoring best practices

## Documentation Updates

### README.md
- Added Security section with quick reference
- Links to SECURITY.md for full policy
- Included cargo audit/update commands
- Updated Contributing section to reference CONTRIBUTING.md

### CHANGELOG.md
- Documented all security-related additions
- Added Security section noting current vulnerabilities
- Tracks new files and configurations

## Current Security Status

GitHub has identified 5 dependency vulnerabilities:
- 1 high severity
- 3 moderate severity
- 1 low severity

Note: cargo update cannot be run in this environment due to network
restrictions, but the documentation provides clear instructions for
users and maintainers to address these issues.

## Automated Security

- Dependabot will now create weekly PRs for updates
- Security vulnerabilities will be flagged immediately
- GitHub Actions updates will be automated
- Reduces manual maintenance burden

This establishes a robust security posture for the project going forward.