v0.5.7: restore subject/delete + plugin-runtime subject helpers

Restores the animus-subject-protocol surfaces wiped by the v0.1.14 "sync from animus-cli" regression and re-adds the animus-plugin-runtime subject helpers downstream Rust plugins depend on.

Restored on animus-subject-protocol 0.1.14 → 0.1.15

• METHOD_SUBJECT_DELETE = "subject/delete" wire constant
• BackendError::Unsupported(String) variant + RpcError mapping to METHOD_NOT_SUPPORTED (-32001)
• SubjectBackend::delete trait method with default impl returning Unsupported
• DeleteSubjectRequest / DeleteSubjectResponse
• Subject::native_status, Subject::status_metadata, Subject::attachments
• SubjectAttachment { id, kind, uri, title, mime_type, metadata }
• StatusDispatchHint { native_status, maps_to, dispatch_label, description }
• SubjectSchema::native_status_values, SubjectSchema::status_dispatch_hints
• SubjectFilter::native_status, dispatch_label, has_attachment_kind
• SubjectChangedEvent::previous_native_status, previous_dispatch_label
• ChangeKind::DispatchLabelChanged, ::AttachmentAdded, ::AttachmentRemoved

Added

• SubjectSchema::supports_delete: bool — mirror of supports_create, defaults to false for back-compat.

animus-plugin-runtime 0.2.0 → 0.2.1

• subject_backend_main(info, backend) — drop-in for the v0.1.13 entrypoint. Auto-derives kind-prefixed <kind>/<verb> aliases from backend.schema().kinds, forwards health/check to backend.health().
• subject_backend_main_with_capabilities(info, backend, extra) — parity with v0.1.13.
• subject_backend_main_with_kinds(info, backend, kinds) — explicit kinds list.
• subject_plugin / subject_plugin_with_kind_aliases — builder-style.
• Plugin::on_health(hook) — backend-specific health/check routing.
• Plugin::advertised_methods() / Plugin::has_method_handler() — test accessors.

Kept from v0.1.14

• schemars::JsonSchema derives on every public message type
• animus-subject-protocol-export-schema bin

Why

The v0.1.14 "sync from animus-cli" merge replaced the upstream protocol crate with a snapshot of the in-tree ao-cli copy, silently dropping a number of subject extensions that downstream plugins depended on. v0.5.7 makes the upstream protocol forward-compatible with v0.1.13 again, so downstream Rust subject plugins can bump their pin to v0.5.7 in a single edit and pick up subject/delete instead of staying trapped on v0.1.13.