refactor: address code review feedback

- Remove TEST_MIGRATION_COMPLETE.md from git tracking (keep local)
- Reorganize task scripts into namespaces:
  - tasks/test-legacy.sh → tasks/test/legacy.sh
  - tasks/check-postgres.sh → tasks/postgres/check_container.sh
- Remove duplicate test:legacy task from mise.toml
- Add DATABASE_URL to mise.toml top-level env
- Fix variable declaration in check_container.sh to follow reset.sh pattern
- Add documentation for test helper functions in SQLx README:
  - search_config_exists() - Check EQL configuration state
  - column_exists() - Verify column presence in schema
  - has_pending_column() - Check encryptindex workflow state

Author: tobyhede