feature: migrate from lerna to npm workspaces

[levivilet]

Summary

• replace Lerna with npm workspaces across the seven packages
• consolidate installation into the root lockfile while preserving all 35 retained direct resolved versions and the internal devcontainer-node workspace link
• update root-hoisted Jest/server paths, cache calculation, workflows, and the all-workspace dependency updater
• retain package-local e2e and memory tool resolutions

Verification

• nice npm ci
• npm run build
• Jest: 7 suites, 30 tests
• npm run type-check
• npm run lint
• changed-file Prettier check
• npm ls --workspaces --depth=0
• direct resolution audit: 35 checked, 1 workspace link, 0 mismatches
• git diff --check

Docker-backed devcontainer e2e is covered by the PR matrix.