chore(release): prepare v2.12.0

[arclabs-studio]

Summary

• Update CHANGELOG.md with v2.12.0 release notes
• Version bump to 2.12.0 (minor — multiple new features since v2.11.0)

Changes Included

Added

• Claude GitHub Actions CI workflows (automated PR review, @claude mentions)
• New skills: arc-final-review, arc-audit, arc-xcode-cloud, arc-worktrees-workflow, arc-memory
• iOS 26 Liquid Glass + @Previewable patterns in arc-presentation-layer
• Skills/skills-index.md comprehensive routing guide
• Security patterns in .gitignore

Changed

• All ARC Labs skills aligned with Anthropic skill guide format
• CLAUDE.md comprehensive rewrite with updated @MainActor per-method patterns, grouped UseCase pattern, multiline formatting, private extension pattern
• Layers/presentation.md and Architecture/mvvm-c.md updated with @Environment DI guidance

Fixed

• arc-spm-manager org URL corrected; ARCPurchasing + ARCAuthentication added

Post-merge steps

1. Merge this PR to main
2. Create annotated tag v2.12.0 on main
3. Create GitHub release
4. Sync develop with main