feat: add environment variable support in migration YAML files

[nlenepveu]

Summary

• Add {env.VAR} syntax to reference environment variables in migration YAML files
• Allows dynamic configuration like cluster names or engine types to be injected from the environment
• Works alongside existing {table} placeholder

Usage Example

production:
  up: |
    CREATE TABLE {table} ON CLUSTER '{env.CLICKHOUSE_CLUSTER}' (
        id UInt64
    ) ENGINE = {env.TABLE_ENGINE}()
    ORDER BY id

Test plan

• Added unit tests for migrate_up with env vars
• Added unit tests for migrate_down with env vars
• All existing tests pass

🤖 Generated with Claude Code