macOS: fold agent control into GatewayConnection

docs/refactor/gateway-client.md

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+# Gateway Client Refactor (Dec 2025)
+
+Goal: remove stringly-typed gateway calls from the macOS app, centralize routing/channel semantics, and improve error handling.
+
+## Progress
+
+- [x] Fold legacy “AgentRPC” into `GatewayConnection` (single layer; no separate client object).
+- [x] Typed gateway API: `GatewayConnection.Method` + `requestDecoded/requestVoid` + typed helpers (status/agent/chat/cron/etc).
+- [x] Centralize agent routing/channel semantics via `GatewayAgentChannel` + `GatewayAgentInvocation`.
+- [x] Improve gateway error model (structured `GatewayResponseError` + decoding errors include method).
+- [x] Migrate mac call sites to typed helpers (leave only intentionally dynamic forwarding paths).
+- [x] Convert remaining UI raw channel strings to `GatewayAgentChannel` (Cron editor).
+- [x] Cleanup naming: rename remaining tests/docs that still reference “RPC/AgentRPC”.
+
+### Notes
+
+- Intentionally string-based:
+  - `BridgeServer` dynamic request forwarding (method is data-driven).
+  - `ControlChannel` request wrapper (generic escape hatch).
+
+## Notes / Non-goals
+
+- 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.