docs(architecture): add ADR-023, resolve OQ-24 — operation error schemas

ADR-023 adds error_schemas to OperationSpec so operations can declare
their domain-level failure modes (FILE_NOT_FOUND, RATE_LIMITED, etc.)
distinct from protocol-level codes (NOT_FOUND, FORBIDDEN, etc.). The
call.error payload gains an optional 'details' field carrying the typed
error payload conforming to the declared schema. from_openapi/to_openapi
map OpenAPI response status codes to/from ErrorDefinitions, making the
adapter contract from ADR-017 faithful on the error axis.

Also fixes W2 (KeyVersionMismatch stale comment in encryption.md —
ADR-021 implements rotation without this variant) and W4
(derive_encryption_key_for_version missing from service.md method list).

Spec updates: operation-registry.md (OperationSpec, ErrorDefinition,
Handler error mapping, services/schema), call-protocol.md (call.error
payload, CallError, ResponseEnvelope), README.md, overview.md,
open-questions.md (OQ-24), call/README.md, encryption.md, service.md.

docs/architecture/overview.md

@@ -213,6 +213,7 @@ All design decisions are documented as ADRs in [decisions/](decisions/).
 | [020](decisions/020-hd-derivation-for-encryption-keys.md) | HD Derivation for Encryption Keys | SLIP-0010 derivation from seed, not PBKDF2; salt field unused in v2 |
 | [021](decisions/021-key-rotation-via-version-indexed-paths.md) | Key Rotation via Version-Indexed Paths | Version-indexed derivation paths; `rotate` re-encrypts between versions |
 | [022](decisions/022-handler-registration-provenance-and-composition-authority.md) | Handler Registration, Provenance, and Composition Authority | Registration bundle carries provenance, composition authority, scoped env, capabilities; dispatch path reads from bundle |
+| [023](decisions/023-operation-error-schemas.md) | Operation Error Schemas | Operations declare domain errors; `call.error` carries typed `details`; adapter fidelity for `from_openapi`/`to_openapi` |
 
 ## Open Questions