chore(deps): update ARCKnowledge submodule — @MainActor guidance (v2.14.0)

[arclabs-studio]

Summary

• Bumps the ARCKnowledge submodule from fb388d2 (v2.13.1) to 6fe1abf (v2.14.0)
• ARCKnowledge v2.14.0 corrects the @MainActor guidance across all skills and reference docs: app targets use @MainActor @Observable final class; Swift packages use nonisolated by default

What changed in ARCKnowledge v2.14.0

• CLAUDE.md Rule feat: migrate to ARCKnowledge #14 — rewritten from "No Blanket @mainactor" to "@MainActor Placement — Match the Module"
• Layers/presentation.md — @mainactor Placement section: app vs package distinction with WWDC 2025-268 / 306 citations
• Architecture/swift-design-principles.md — Principle 5 code examples updated
• arc-presentation-layer/SKILL.md + reference — class-level @MainActor, let viewModel
• arc-swift-architecture/SKILL.md + reference — same pattern
• arc-audit/SKILL.md — removed incorrect grep check; updated checklists
• Skills/skills-index.md — version entry updated

Test plan

• arcdevtools-setup still works correctly in a consumer project (skills symlinked)
• ARCKnowledge skills load cleanly after setup
• No regression in existing hooks or config files

Refs: FavRes FVRS-182 / ARCKnowledge #83