feat: data quality, schema enforcement, DAG viz, versioning, tests, release docs

[rafafrdz]

Summary

• Data quality assertions (Add data quality checks and assertions in pipeline YAML #106): Assertion transformation with not_null, expression, and row-count checks. Configurable onFailure: fail/warn/log
• Schema enforcement (Add schema evolution and enforcement in YAML #107): SchemaEnforce transformation with strict/permissive/evolve modes. Column type casting, nullable enforcement, default values
• Pipeline versioning (Add pipeline versioning and migration support #108): version field in ETL, Migration engine with migration rules and deprecation warnings
• DAG visualization (Add pipeline DAG visualization as ASCII/Mermaid/DOT output #109): --graph mermaid|dot|ascii CLI flag via GraphViz utility
• Release automation (Publish teckel to Maven Central with proper release automation #110): CONTRIBUTING.md with dev setup, test commands, and release process documentation
• Comprehensive tests (Add comprehensive unit and integration test suite #101): 53 unit tests across serializer (VariableResolver, SecretResolver, ConfigMerger, Validator) and api (DryRun, DocGen)

Closes #101, #106, #107, #108, #109, #110

Test plan

• sbt clean compile passes
• 53 new unit tests pass
• CI green