feat(cre): source tree skeleton with empty module dirs and CI workflow
- src/cre.cr entry point + version.cr
- Empty module dirs: cli, tui, engine, events, rotators, policy, audit, crypto, persistence, notifiers, compliance, config, domain, aws, vault, github, demo
- spec/ unit + integration + fixtures structure mirroring src/
- spec_helper.cr with stdlib spec + cre require
- GitHub Actions CI: format, unit matrix (Crystal 1.19/1.20/nightly x ubuntu/macos), integration with PG service, build job
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