chore: remove incomplete add_hierarchy_support migration and add test-infrastructure spec

Remove generated Prisma migration files (migration.sql and rollback.sql) for
20251031000000_add_hierarchy_support to avoid applying incomplete schema
changes. Add specs/20251102/001-test-infrastructure-improvements/README.md
with a proposal and implementation plan for improving test infrastructure:
database lifecycle utilities, test data factories, auth service mocking,
Vitest configuration, CI integration, and documentation.

Author: tikazyq