Add Swift Testing modernization skill

[sreejithraman]

What changed

• Add a standalone swift-testing-modernization skill for existing Swift unit and integration test suites.
• Route XCTest migrations and current Swift Testing cleanup through focused references.
• Preserve failure control, lifecycle cleanup, async deadlines, toolchain compatibility, and parallel execution behavior.
• Keep UI tests, performance tests, and unsupported migration cases in XCTest.

Why

This keeps the useful Swift Testing modernization workflow available without retaining the broader iOS plugin or duplicating the existing TDD and debugging skills.

Effect

Agents can migrate a suite in small slices, use modern Swift Testing features when the project supports them, and leave deliberate XCTest holdouts when a migration would change behavior.

Verification

• python3 scripts/check_skills.py
• git diff --check
• python3 /Users/sree/.codex/skills/.system/skill-creator/scripts/quick_validate.py skills/swift-testing-modernization
• Five independent native and Gemini review rounds; final round clean
• Primary documentation links checked successfully