mnm-core — shared primitives for the midnight-manual workspace.
Phase 2 lands the foundational shared types: typed error envelope
([error]), embedding-model wire id ([model_id]), provenance / content
/ source / chunk types ([provenance], [types]), XDG config discovery
([config]), auth-file reader ([auth_file]), and the scoring-policy
loader ([scoring_policy]). Each module is independently usable and
independently testable.