Remove Lerna dependency from package.json

[Streamweaver]

Context

Phase 2 of the TOAST UI Editor fork upgrade plan.

Task

Remove lerna (^3.20.2) from root package.json devDependencies. This eliminates a large vulnerable dependency tree.

Also remove any lerna.json configuration file if present.

Acceptance Criteria

• lerna no longer in devDependencies
• No script or config references lerna
• npm install succeeds without Lerna

Plan: UPGRADE_PLAN.md (Phase 2)