chore: add prettier, changesets, and Spindle CI workflows
- Add prettier for linting with lint/lint:fix scripts
- Add @changesets/cli for version management and npm publishing
- Add .tangled/workflows/ci.yaml (lint, typecheck, test on push/PR)
- Add .tangled/workflows/publish.yaml (changeset version + publish on main)
- Format codebase with prettier
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>