Add SwiftLint build phase to SwiftUI App template

[ssestak]

Summary

Adds a SwiftLint Run Script build phase to the SwiftUI App Xcode project template. Generated projects autocorrect on every build and surface remaining warnings in Xcode's issue navigator — no manual install of the phase itself needed after project creation.

Stacked on #64. PR base will retarget to main automatically once #64 merges.

Flow

flowchart LR
    A[Build starts] --> B[Compile Sources]
    B --> C[Link Frameworks]
    C --> D[Copy Resources]
    D --> E[SwiftLint phase]
    E --> F{swiftlint on PATH?}
    F -- yes --> G[swiftlint --fix --quiet]
    G --> H[swiftlint]
    H --> I[Warnings in Xcode]
    F -- no --> J[Build-log warning:<br/>'brew install swiftlint']

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Key Changes

• Templates/SwiftUI App.xctemplate/TemplateInfo.plist
  • New BuildPhases array on the app target with a single ShellScript phase named SwiftLint.
  • ENABLE_USER_SCRIPT_SANDBOXING = NO added to SharedSettings — Xcode 15+ sandboxes script phases by default, which blocks SwiftLint from reading project files (Sandbox: swiftlint deny file-read-data …).
• Templates/SwiftUI App.xctemplate/App.swift — new #warning instructing the developer to drag the SwiftLint phase above Compile Sources.
• Script exports /opt/homebrew/bin for Apple Silicon, runs swiftlint --fix --quiet then swiftlint. Graceful fallback emits a build-log warning when SwiftLint isn't installed.

Known Limitation — Phase Position

Xcode project templates can only append custom phases to a target — there is no documented key (InsertBefore, Position, ReplaceBuildPhases, …) to insert before phases inherited from com.apple.dt.unit.applicationBase. Confirmed empirically (declaring Sources/Frameworks/Resources in the child array creates duplicate empty phases instead of replacing the inherited ones) and via research of public Xcode templates on disk and community projects (surfstudio, GoodRequest) — all of them rely on the same after-Sources position.

Consequence: as generated, the SwiftLint phase runs after Compile Sources, so --fix corrections apply to the next build. The new #warning in App.swift tells the developer to drag the phase above Compile Sources once — the reorder persists in project.pbxproj.

Out of Scope

• Shipping a default .swiftlint.yml inside the template — left to the developer.
• Build phase for the Scene file template — file templates can't inject build phases.
• Programmatic post-creation reordering of project.pbxproj — would require shipping a Ruby/Python script with the template.

Test Plan

• Generate a project from the SwiftUI App template with SwiftLint installed; confirm the SwiftLint build phase appears on the target (one, not duplicated)
• Introduce a deliberate style violation; confirm it appears as an Xcode warning
• Manually drag the SwiftLint phase above Compile Sources, introduce an autocorrectable violation; confirm --fix corrects it on the same build
• Generate on a machine without SwiftLint; confirm the build succeeds with the install-hint warning
• Confirm no Sandbox: swiftlint deny … errors in the build log
• Confirm the #warning about reordering appears in the issue navigator on first build

[jmarek41]

Pushed 47ababb applying the review comments so the template matches Futrumi and iRozhlas:

• Added a SwiftFormat run script phase, appended before SwiftLint so formatting runs first.
• Both scripts now skip Xcode preview builds (BUILD_DIR contains Previews), prepend /opt/homebrew/bin only when present, and use swiftlint --fix --quiet.
• Missing SwiftLint is reported as error: (as in the handbook and both projects); missing SwiftFormat as warning:.
• The #warning in App.swift now asks to drag both phases above Compile Sources, in that order.

Handbook counterpart: futuredapp/Engineering-Handbook#119.