ci: refactor CI/release trigger model and dedupe branch automation

[iamfj]

Summary

• make CI workflow PR-only for next/main and keep canonical required checks on Node 22 (Unit tests on node v22, Code checks on node v22)
• add lightweight CI Post-merge Sentinel workflow for pushes to next/main (build + packed binaries)
• refactor release workflow to run on push to next/main + manual dispatch, remove schedule, and remove duplicate test/lint/typecheck steps
• dedupe promotion automation by removing release trigger and adding no-op guard when next has no commits ahead of main
• add/update CI run-model docs (docs/ci-run-model.md, README, CONTRIBUTING) to match new behavior

Test Plan

• npm run check:ci
• npm run generate
• npx tsc --noEmit
• npm test
• npm run build
• Verified workflow triggers:
  • release-check.yml has no schedule
  • promote-next-to-main.yml has no release trigger
• Verified ruleset required checks remain:
  • Unit tests on node v22
  • Code checks on node v22