Gateway Client Refactor (Dec 2025)

Goal: remove stringly-typed gateway calls from the macOS app, centralize routing/channel semantics, and improve error handling.

Progress

Fold legacy “AgentRPC” into GatewayConnection (single layer; no separate client object).
Typed gateway API: GatewayConnection.Method + requestDecoded/requestVoid + typed helpers (status/agent/chat/cron/etc).
Centralize agent routing/channel semantics via GatewayAgentChannel + GatewayAgentInvocation.
Improve gateway error model (structured GatewayResponseError + decoding errors include method).
Migrate mac call sites to typed helpers (leave only intentionally dynamic forwarding paths).
Convert remaining UI raw channel strings to GatewayAgentChannel (Cron editor).
Cleanup naming: rename remaining tests/docs that still reference “RPC/AgentRPC”.

Intentionally string-based:
- BridgeServer dynamic request forwarding (method is data-driven).
- ControlChannel request wrapper (generic escape hatch).

No functional behavior changes intended (beyond better errors and removing “magic strings”).
Keep changes incremental: introduce typed APIs first, then migrate call sites, then remove old helpers.