set-up-ci: add Swift / iOS (XcodeGen + xcodebuild) CI template and detection

[cboone]

Summary

set-up-ci has no Swift / iOS template. The detection table (skills/set-up-ci/SKILL.md, step 1) lists Go, JS/TS, Python, Rust, Ruby, Shell, Zig, Zsh, but not Swift, and there is no references/ci-swift.md / references/makefile-swift.md. A Swift app therefore falls through to the generic make test / make lint workflow, which is useless for an iOS app (no SwiftPM; build is xcodebuild).

Discovered while bootstrapping an iOS app repo (XcodeGen + xcodebuild).

Suggested addition

Add a Swift detection row and a Swift CI template. Two sub-cases:

• iOS/macOS app (XcodeGen + xcodebuild): needs a macOS runner. Steps: install/run xcodegen generate (if project.yml present), then xcodebuild build and xcodebuild test against a simulator destination. Example core:

  runs-on: macos-latest
  steps:
    - uses: actions/checkout@<sha>
    - run: brew install xcodegen   # or mint, if project.yml present
    - run: xcodegen generate
    - run: xcodebuild -scheme <Scheme> -sdk iphonesimulator \
        -destination 'platform=iOS Simulator,name=iPhone 17 Pro' build
    - run: xcodebuild test -scheme <Scheme> -sdk iphonesimulator \
        -destination 'platform=iOS Simulator,name=iPhone 17 Pro'

• Swift package (SwiftPM): swift build / swift test, can run on macos-latest (or swift:linux containers for cross-platform libs).

Caveats to document

• macOS runners cost ~10x Linux minutes; call this out (ties into optimize-runner-usage), especially for private repos.
• Simulator destination/runtime drift: recommend xcodebuild -downloadPlatform iOS or a runtime-pinning step.
• Scheme name is project-specific; read it from project.yml or xcodebuild -list.
• A version file analogous to the other templates: .swift-version (already produced by set-up-linters).

If a full macOS CI template is out of scope, at minimum add Swift to the detection table and have the skill explicitly report "no Swift CI template; macOS runner + xcodebuild required" instead of silently emitting the generic make workflow.