Adds:
- docs/branching.md — beginner contributor guide
- docs/adr/XXXX-...md — ADR (will be renamed with issue#)
- .github/workflows/auto-backmerge.yml — disabled in Phase 1
- .github/workflows/pr-target-validator.yml — warn-only in Phase 1
- scripts/setup-branch-protection.sh — idempotent gh api script
Modifies:
- .github/workflows/branch-naming.yml — recognize 'next'
- CONTRIBUTING.md — 'Where Do I Open My PR?' section
Phase 1 is additive: nothing operational changes until Phase 2 flips
auto-backmerge.yml's if:false→true, flips pr-target-validator.yml's
WARN_ONLY→false, creates the next branch, and switches the default
branch. See the ADR for the migration plan.