[codex] Fix release workflow and brand overrides

[nahiyankhan]

Summary

• pin the release workflow to Node 18 so the nested UI install avoids the Node 24/npm 11.13 exit-handler failure seen on main
• merge brand override parsing with enforcement support, legacy severity compatibility, and warnings for unknown or invalid override fields
• backfill brand-grouped platform builds for every configured brand and cover the resolver/pipeline behavior with tests

Validation

• pnpm exec biome check .
• pnpm exec tsc --noEmit
• pnpm exec vitest run
• npm ci --prefix src/dev/ui --loglevel=warn
• pnpm build
• pre-commit hook: pnpm biome check .
• pre-push hook: pnpm vitest run, pnpm biome check ., pnpm tsc --noEmit