Use LucaTools/select-xcode action

[albertodebortoli]

Description

Use LucaTools/select-xcode@v1 action and remove local copy

Description

• Replaces the local composite action .github/actions/select-xcode/action.yml with the published LucaTools/select-xcode@v1 action across all three macOS workflows.
• The local action is now maintained as a standalone reusable action in the LucaTools/select-xcode repository; keeping a copy here is redundant and creates drift risk.
• All usages pass no explicit inputs and continue to rely on the .xcode-version file in the repo root — no behaviour change.

Type of Change

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

How Has This Been Tested?

• Added / updated unit tests
• Manually tested locally (describe)
• Tested on macOS (arch: arm64 / x86_64)
• Other — relies on the existing CI run triggered by this PR to validate the external action resolves and selects Xcode correctly on macos-15

Screenshots / Demo (if applicable)

N/A

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 (describe impact and migration path)

Additional Notes

The .github/actions/ directory has been removed entirely. The three affected steps (in run-tests.yml, docc.yml, and publish-release.yml) now reference LucaTools/select-xcode@v1 directly.