Add SwiftLint configuration and CI enforcement

[NachoSoto]

Summary

• add a shared SwiftLint configuration and instructions for contributors to run it locally
• replace the force-unwrapped file operations in the disk cache implementation with safer error handling so the lint passes without suppressing the rule
• extend the build-and-test workflow to install SwiftLint and run it with the GitHub Actions reporter after the test suite

Testing

• ./.tools/swiftlint --config .swiftlint.yml

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Codex Task