Verify package-scoped build/test/lint after Lerna removal

[Streamweaver]

Context

Phase 2 of the TOAST UI Editor fork upgrade plan.

Task

Verify that all package-scoped and aggregate commands work correctly after Lerna removal:

• Run package-scoped lint command
• Run package-scoped test command
• Run package-scoped build command
• Run aggregate lint:all, test:all, test:types:all, build:all

Acceptance Criteria

• All aggregate commands succeed
• Package-scoped commands succeed for all 9 packages
• No local or CI script depends on lerna

Plan: UPGRADE_PLAN.md (Phase 2)