v0.18.0

Added

• Added the AppCode::UserAlreadyExists classification and mapped it to RFC7807
  responses with the appropriate retry hint.

Changed

• Switched all integration converters in src/convert/* to build structured
  Context metadata before producing Error values, including HTTP status,
  operation, endpoint, duration and retry/edit flags.
• Extended integration tests to validate the enriched metadata, retry behavior
  and error code/category mappings across the updated converters.