chore: declare npm toolchain - #29

Summary

• declare Sentinel's npm 11 package manager metadata
• keep package-lock root engines in sync with package.json
• update docs/TOOLCHAIN.md to identify npm@11.14.1 as the canonical package manager

Verification

• JSON parse check for package.json and package-lock.json
• git diff --check
• mise exec node@24.16.0 -- npm test
• mise exec node@24.16.0 -- npm run build
• mise exec node@24.16.0 -- npm run quality:react-doctor

Release/deploy: not applicable for metadata/docs-only publish.