Add security manual and reporting guidelines

[Josephat-S]

This pull request adds a comprehensive SECURITY.md file to the repository, outlining the project's security policy, supply chain security measures, dependency management, and security features in generated projects.

Key additions in the new security policy documentation:

Security Policy and Reporting:

• Provides clear instructions for responsibly reporting vulnerabilities via a dedicated security email, including response timelines.

Supply Chain and Dependency Security:

• Details the CLI's minimal dependency footprint, network access policy (only to GitHub for templates), and justifies the presence of dynamic code execution in certain dependencies as low risk.
• Documents the handling of deprecated and transitive dependencies, including plans for future upgrades and current auditing practices (e.g., npm audit in CI).

Security Features in Generated Projects:

• Lists security hardening features included in generated projects when enabled (e.g., environment validation, CSP headers, secure .gitignore, and .env.example practices).

Supported Versions:

• Specifies which CLI versions are actively supported.

Closes #81