Add SwiftLint configuration and CI lint job

[jsattler]

Summary

• Add .swiftlint.yml with a curated set of rules scoped to BetterCapture/ and BetterCaptureTests/
• Add a lint job to pull-request.yml that runs SwiftLint with inline PR annotations via --reporter github-actions-logging

Changes

• .swiftlint.yml — new SwiftLint config with opt-in rules, disabled todo rule, and line length thresholds
• .github/workflows/pull-request.yml — new lint job running before build and test