Summary

• preserve todos when a later thread state update omits the todos field
• keep explicit empty todo lists as a valid clear operation
• add regression tests for omitted-vs-empty todo state updates

Verification

• pnpm exec vitest run tests/unit/core/threads/message-merge.test.ts
• pnpm exec eslint src/core/threads/hooks.ts tests/unit/core/threads/message-merge.test.ts
• pnpm typecheck

Closes #3271