Implement spec file discovery

[albertodebortoli]

Pull Request Title

If Lucafile is present, it is used by default even if there are other Lucafiles (e.g. Lucafile-linux).
If multiple Lucafiles are present but not one with the exact name "Lucafile", the user is prompted to pick one:

Type of Change

• Feature
• Bug fix
• Maintenance / Refactor
• Documentation
• CI / Tooling
• Other (specify)

How Has This Been Tested?

Describe the testing strategy and commands executed.

• Added / updated unit tests
• Manually tested locally
• Tested on macOS (arch: arm64 / x86_64)
• Other

Screenshots / Demo (if applicable)

Add images, logs, or terminal output.

Checklist

• Swift code builds locally (swift build)
• Tests pass locally (swift test)
• Code style / formatting respected
• Documentation updated (README / comments)
• Version / tag alignment considered (if release related)
• PR title follows conventional style (optional)

CI Considerations

• Affects build time notably
• Requires new secrets / env vars
• Alters release process

Breaking Changes?

• No
• Yes