[Deploy] CI/CD pipeline — build, test, security scan, Docker images

[maxzaikin]

Summary

Create complete GitHub Actions CI/CD pipeline.

Context

See Contributing-Guide

Acceptance Criteria

• .github/workflows/build.yml:
  • Trigger: push to develop/main, PR to develop/main
  • Job: build — dotnet restore + build (all projects)
  • Job: test — dotnet test VaultFlower.UnitTests
  • Job: security — CodeQL analysis for C#
  • Job: deps — OWASP Dependency-Check
  • Job: secrets — secret scanning (no hardcoded tokens)
  • Job: integration — spin up docker-compose.infra + run integration tests
  • Job: docker — build all images + Trivy CVE scan
  • Job: push — push to private registry (on main only)
• .github/workflows/release.yml:
  • Trigger: git tag v*
  • Build + test + security (same as build.yml)
  • Create GitHub Release with CHANGELOG entry
  • Push versioned images to registry
• All jobs must pass before PR can merge (branch protection)
• Verified: pipeline passes on clean develop branch