[emmanuelgautier]

Fixes #79

Problem

The Kratos connector copied user_metadata into the identity's traits after setting email, username and phone from the user's identifiers, so a user_metadata key with one of those names silently overwrote the identifier trait.

Change

• buildTraits now copies user_metadata first and sets the identifier traits afterwards, so the user's own identifiers win. The colliding value is still kept in metadata_public.
• validate reports a user_metadata key named email, username or phone for Kratos, and points to a metadata: mapping route to move or drop it. The check applies with or without --schema-file.
• Docs updated (validate reference and metadata mapping section).

Tests

Unit tests cover the precedence in buildIdentity and the new validation rule (including that app_metadata is unaffected). go test ./pkg/connector/kratos/ and go vet ./... pass; the live Kratos test was not run.