refactor(chat): route browser chat through one runtime

Co-Authored-By: Claude Opus 4.8 <[email protected]>
Claude-Session: https://claude.ai/code/session_01ESFAnh2t9HmLwng8oW95St

packages/types/src/chat/events.ts

@@ -150,8 +150,28 @@ export type HarnessTurnAckPayload =
       readonly selection?: HarnessSelection;
     };
 
+/**
+ * The frozen browser send-protocol advertisement (Task Five; server → client only).
+ *
+ * A conversation id or a harness selection never proves that the connected Gateway actually
+ * handles a given wire event, so after BetterAuth authenticates a browser Socket connection the
+ * Gateway advertises — exactly once, targeted to that socket — which send event the client may
+ * use. `legacy-message` in legacy mode, `unavailable` in `pi-rpc` (including test-ready Pi
+ * graphs); Task Five never advertises `turn-send` (its authenticated handler lands in Task 15).
+ * Capability is routing information, never authorization: every server handler still enforces
+ * authentication, ownership, DTO, mode, and runtime checks.
+ */
+export type ChatSendProtocol = 'legacy-message' | 'turn-send' | 'unavailable';
+
+export interface ChatSendCapabilityPayload {
+  readonly protocol: ChatSendProtocol;
+  /** Exact Socket.IO id for the authenticated browser connection this advertisement is bound to. */
+  readonly connectionId: string;
+}
+
 /** Socket.IO typed event map: server → client */
 export interface ServerToClientEvents {
+  'chat:send-capability': (payload: ChatSendCapabilityPayload) => void;
   'message:ack': (payload: MessageAckPayload) => void;
   'agent:start': (payload: AgentStartPayload) => void;
   'agent:end': (payload: AgentEndPayload) => void;

packages/types/src/chat/index.ts

@@ -16,6 +16,8 @@ export type {
   ChatMessagePayload,
   HarnessTurnSendPayload,
   HarnessTurnAckPayload,
+  ChatSendProtocol,
+  ChatSendCapabilityPayload,
   ServerToClientEvents,
   ClientToServerEvents,
 } from './events.js';