feat: add swift language support to code simplifier

[kostyay]

• Register .swift file extension in the simplify extension module for automatic language routing
• Add comprehensive Swift code simplifier skill covering Swift 6+ idioms including async/await, actors, and structured concurrency
• Document naming conventions following Apple's API Design Guidelines with clarity-at-point-of-use as the primary goal
• Include detailed refinement patterns for guard statements, optionals, enums, error handling, protocol-oriented design, and access control
• Provide practical examples for modern Swift idioms: trailing closures, implicit returns, key paths, pattern matching, and value types
• Document skill usage guidelines, verification process via swift build/swift test/linters, and the principle of preserving functionality while improving clarity