Protect main by repository settings alone; drop main-source-guard #64

The maintainer's decision (2026-10-03, relayed by the ecosystem lead): main is protected by repository settings only (option B): pull request required, 0 approving reviews, no direct push, force-push or deletion, not locked, and merging limited to administrators. The maintainer merges the release PR after checking its source branch. Done when main-source-guard.yml is gone, release-sync.yml no longer posts the guard status (step and statuses: write), protect-main.sh applies the B settings (as python-multiplatform's PR #122 does), and the release README, AGENTS.md and PROJECT.md describe it. Applying the settings is the maintainer's own step.