feat(container): add devrail-init.sh progressive adoption script (#11)

Standalone shell script that scaffolds DevRail configuration files
in any project through 4 progressive layers:

1. Agent instruction files (CLAUDE.md, AGENTS.md, .cursorrules, .opencode/)
2. Pre-commit hooks (language-aware .pre-commit-config.yaml)
3. Makefile + container (.devrail.yml, Makefile, DEVELOPMENT.md)
4. CI pipeline (GitHub Actions or GitLab CI)

Features:
- Interactive and non-interactive modes (--yes, --force)
- Dry-run support (--dry-run)
- Language-aware config generation from .devrail.yml
- Makefile merge strategy (backup + include for existing Makefiles)
- .gitignore append-with-marker pattern
- Idempotent — safe to re-run
- 16 bats tests covering all modes and edge cases

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: matthew-on-git