main protection by settings only (option B): drop main-source-guard #121

Maintainer decision (2026-10-03, option B): main is protected by branch settings only — PR required, no approving review, no direct/force push, no deletion, not locked, only administrators merge; the maintainer merges the release PR and checks its head. The source-branch workflow and the status release-sync posted for it are removed. Completion: no main-source-guard.yml, release-sync posts no status, tools/release/protect-main.sh applies the B settings.