…migration

This commit establishes the full CI/CD setup for the Branch Newspaper repository
migration from GitLab to GitHub, with bi-directional mirroring support.

## CI/CD Infrastructure
- Add GitHub Actions CI workflow with Elixir test matrix (1.15.0-1.16.0)
- Add mirror-sync workflow for event-driven GitHub → GitLab mirroring
- Update .gitlab-ci.yml to use shared scripts for consistency
- Configure Dependabot for Elixir, npm, and GitHub Actions updates

## Shared CI Scripts (ci-scripts/)
- setup.sh: Environment setup and dependency installation
- test.sh: Test execution with coverage support
- lint.sh: Code quality checks (format, compile warnings, credo)
- build.sh: Release building for deployment
- verify-mirror.sh: Mirror synchronization verification
- mirror-push.sh: GitLab push with retry logic

## Documentation
- README.md: Comprehensive setup guide with badges and project structure
- SECRETS.md: Complete secrets configuration guide
- TODO.md: Prioritized task backlog from codebase analysis
- ROADMAP.adoc: MVP v1.0 deployment roadmap with architecture

## Key Features
- Test matrix across Elixir 1.15.0, 1.15.7, 1.16.0
- Parallel test execution
- Dependency caching for faster CI runs
- Automatic mirror sync on push events
- CodeQL security scanning (existing)
- Branch protection recommendations documented