[CI/CD] Add comprehensive GitHub Actions workflows for testing, linting, security, and deployment

[leanworld7-netizen]

CI/CD GitHub Actions Workflows (Closes #22)

Implements comprehensive CI/CD per issue #22:

Workflows Added

1. ci.yml - Continuous Integration

   • Lint & Format (rustfmt + clippy with -D warnings)
   • Unit Tests (cargo test)
   • Security Checks (cargo audit + unsafe code scan)
   • Build Matrix (utility_contracts + price_oracle)

2. deploy.yml - Continuous Deployment

   • Triggered on version tags (v*)
   • Deploys to Stellar testnet via soroban-cli
   • Deployment summary in GitHub Actions

3. dependency-review.yml - Dependency Security

   • Runs on every PR
   • Fails on moderate+ severity vulnerabilities
   • Posts summary comment in PR

Security Features

• Cargo audit for known vulnerabilities
• Unsafe code detection in production contracts
• Dependency review on all PRs
• Clippy with warnings-as-errors