chore(release): prepare v2.9.0

[arclabs-studio]

Release v2.9.0

Summary

• Introduces 12 ARC Labs subagents for autonomous task execution
• Closes the full development workflow gap (Linear → code → review → PR → release → TestFlight → ASO)
• Adds safe-by-default agents for high-risk operations (SwiftData migration, SPM management)

Changes Included

New Agents (.claude/agents/)

• arc-swift-tdd — TDD implementation, writes tests before code (sonnet)
• arc-swift-reviewer — Delegated code review, structured report (sonnet)
• arc-swift-debugger — Build/test failure diagnosis, environment-first (sonnet)
• arc-spm-manager — Package.swift management and verification (haiku)
• arc-xcode-explorer — Read-only codebase navigation and mapping (haiku)
• arc-linear-bridge — Linear ticket to Swift test scaffolding + branch creation (haiku)
• arc-pr-publisher — Validates pre-PR checklist, creates PR, links Linear ticket
• arc-release-orchestrator — Bumps version, updates CHANGELOG, creates release PR
• arc-testflight — Orchestrates archive → upload → TestFlight configuration
• arc-aso — App Store Optimization, produces ready-to-upload metadata
• arc-swiftdata-migration — High-risk schema migration, test-before-code
• arc-dependency-auditor — Read-only SPM dependency audit

Updated

• arc-linear-bridge — Added branch creation via github_create_branch
• arc-spm-manager — Fixed org URL; added ARCPurchasing and ARCAuthentication
• Skills/skills-index.md — Added Agents routing table
• CHANGELOG.md — v2.9.0 entry

Post-merge Steps

1. Merge this PR
2. Tag v2.9.0 on main: git tag -a v2.9.0 -m "Release v2.9.0"
3. Push tag: git push origin v2.9.0
4. Create GitHub Release from the tag
5. Sync develop with main