- New arc-tracker crate defines the Tracker trait and shared Issue/BlockerRef types
- arc-linear: replace Issue/BlockerRef with arc-tracker re-exports, add LinearTracker struct implementing Tracker, remove free functions
- arc-github: add GitHubTracker implementing Tracker via Projects V2 GraphQL API, with lazy project node ID resolution (org → user fallback) and create_installation_access_token_for_projects helper
- 47 tests in arc-github (15 new), 24 tests in arc-linear (updated to use LinearTracker)
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>