Introduce ResponseEnvelope as a first-class concept for uniform operation
result handling across MCP, OpenAPI, and local adapters. Key decisions:
- ResponseEnvelope<T> wraps every result with typed metadata
(LocalResponseMeta, HTTPResponseMeta, MCPResponseMeta)
- CallHandler becomes sole publisher of call.responded (handler
responsibility shift from publish-to-return)
- Envelope detection via closed-set string discriminant
(isResponseEnvelope) — no Symbols, JSON-serializable
- MCP isError:true no longer throws; wraps in envelope with
meta.isError flag preserving error content for consumers
- outputSchema validates envelope.data, not the full envelope
- PendingRequestMap.respond() validates envelope at runtime
- Factory functions (localEnvelope, httpEnvelope, mcpEnvelope)
ensure consistent construction
- Breaking change: execute() returns ResponseEnvelope<TOutput>,
call.responded.output is ResponseEnvelopeSchema
- No Client abstraction yet (ADR-014)