Document SwiftLint and SwiftFormat prerequisites

Developers building Peekaboo from source could hit avoidable setup failures because the build documentation omitted the Swift linting and formatting tools required by the validation scripts.

Added SwiftLint and SwiftFormat to the documented source-build prerequisites with the Homebrew install command, without changing build scripts, lint configuration, or release behavior.

• Reduces onboarding friction for macOS contributors building Peekaboo from source.
• Makes local validation expectations explicit before contributors run pnpm helper scripts.
• Keeps the fix low risk by limiting the change to documentation while preserving existing tooling behavior.

• Verified the documentation change with the repository docs lint script.
• Confirmed the diff is whitespace-clean with git diff --check.
• Merged upstream and closed the linked prerequisite documentation issue.