feat: Enhanced certificate handling, improved ergonomics, and documentation

[nsakaimbo]

Summary

This PR consolidates enhancements across three key areas: documentation, certificate handling, and action ergonomics.

Documentation

• New apple_codesigning_overview.md: Comprehensive guide explaining Apple code signing concepts, certificate types, provisioning profiles, and common workflows
• Updated README.md: Improved usage examples and clearer documentation of action inputs/outputs

Enhanced Certificate Handling

P12 and PEM Format Support

• Support for both P12 (PKCS#12) and PEM certificate formats
• Automatic format detection based on certificate content

Encrypted Private Key Support

• Handle password-protected private keys seamlessly
• Secure passphrase handling during certificate import

Smart Base64 Detection

• Intelligent detection of base64-encoded vs raw certificate data
• Eliminates need for users to manually specify encoding format
• Graceful handling of certificates with or without base64 encoding

Simplified P12 to PEM Conversion

• Refactored to use OpenSSL exclusively for P12 to PEM conversion
• Removed unused importCertificateFromP12Buffer method for cleaner codebase

Action Ergonomics

Default Combined Setup Mode

• No longer requires explicit asset-type input
• Action now defaults to combined certificate + provisioning profile setup
• Reduces boilerplate configuration for common use cases

Automatic Keychain Password Generation

• Auto-generates secure keychain passwords when not explicitly provided
• Improves security by encouraging unique passwords per workflow run

Some CI Fixes

• Resolved super-linter CI errors
• Fixed markdown blockquote formatting issues
• Corrected API key validation path in CI tests
• ESLint/Prettier checks are clean

Testing

npm run lint      # ✅ Pass
npm run format:check  # ✅ Pass
npm test          # ✅ 22/22 tests pass