YAML-only mode: define features without writing Python

[gauthierpiarrette]

Timefence supports timefence.yaml for configuration, but feature definitions with custom SQL or transforms still require Python. A fully declarative YAML mode would let users define everything in config:

sources:
  users:
    path: data/users.parquet
    keys: [user_id]
    timestamp: updated_at

features:
  country:
    source: users
    columns: [country]
  spend_30d:
    source: transactions
    embargo: 1d
    sql: |
      SELECT user_id, created_at AS feature_time,
             SUM(amount) OVER (...) AS spend_30d
      FROM {source}

This would make Timefence usable from pure CLI workflows without any Python.