Add Swift package compiler settings parity

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Summary

• add a SwiftPM-native compiler-settings baseline aligned with the applicable Xcode warning, language-mode, concurrency, and upcoming-feature policy
• extend the completion audit and structural validator to enforce package manifests and future Xcode-to-SwiftPM parity maintenance
• explicitly exclude package plug-in targets that cannot express swiftSettings and reject every redundant Swift 6 StrictConcurrency opt-in spelling
• add focused mutation tests and prepare release 0.0.33

Validation

• Scripts/swift_format.sh format-and-lint Scripts Tests .agents/skills/agent-guidelines-audit/scripts
• Scripts/validate_guidelines.swift — 20 guidelines validated for 0.0.33
• Tests/run_tests.swift — 31 tests passed
• .agents/skills/agent-guidelines-audit/scripts/check_markdown_wrapping.swift README.md CHANGELOG.md Guidelines .agents/skills/agent-guidelines-audit/SKILL.md — 23 files passed
• git diff --check
• completion audit: clean; repository contains no checked-in Package.swift or .xcodeproj consumer target requiring a build
• Xcode 27.0 / Swift 6.4 installed PackageDescription interface confirms treatAllWarnings(as:) for Swift/C/C++ settings and defaultIsolation(_:) are available from PackageDescription 6.2; Target.plugin(...) exposes no swiftSettings

Review remediation

• resolved the initial exact-head ChatGPT review's plug-in-target P1 by scoping the policy to targets that support compiler settings and pinning the exclusion structurally
• resolved the StrictConcurrency P1 by rejecting and structurally/mutationally covering upcoming-feature, experimental-feature, and StrictConcurrency=complete forms