Restructure `cli` and `tui` with shared `lib` crate

[jorgenosberg]

The TUI has gained some attention and should be an available interface option moving forward. But instead of duplicating logic between repos, both interfaces should be grouped together in a new repo openapi-validator with 3 crates and 2 Formulae:

oav-lib: shared crate for logic
oav-cli: CLI
oav-tui: lazyoav TUI

Basically, the migration should be:

1. Create entur/openapi-validator repo
2. git subtree add both existing repos (preserves full history)
3. Restructure into workspace layout
4. Extract shared code into oav-lib incrementally (config → docker → pipeline → generators → spec → scaffold)
5. Update release workflow to build both binaries
6. Add lazyoav.rb formula
7. Archive old repos with a redirect notice in the README

An exact copy of this issue will be available in the TUI repo: https://github.com/entur/openapi-validator-tui/issues