feat(gateway): add WebSocket gateway with JSON-RPC protocol and auth

Phase 2 of the Flynn roadmap. Adds a WebSocket gateway server that
starts alongside the Telegram bot, providing real-time API access to
the agent, sessions, and tools.

Protocol: JSON-RPC-like (request/response/event) over WebSocket.
8 methods: agent.send, agent.cancel, sessions.list, sessions.history,
sessions.create, tools.list, tools.invoke, system.health.

Auth: Bearer token + Tailscale identity header support.
Session bridge: per-connection agent instances with shared model router.

New files: src/gateway/ (protocol, router, server, auth, session-bridge,
handlers for agent/sessions/tools/system).
57 new tests (181 total), typecheck clean.

src/gateway/handlers/sessions.ts

@@ -0,0 +1,59 @@
+import type { GatewayRequest, OutboundMessage } from '../protocol.js';
+import { makeResponse, makeError, ErrorCode } from '../protocol.js';
+import type { SessionManager } from '../../session/manager.js';
+
+export interface SessionHandlerDeps {
+  sessionManager: SessionManager;
+}
+
+export function createSessionHandlers(deps: SessionHandlerDeps) {
+  return {
+    'sessions.list': async (request: GatewayRequest): Promise<OutboundMessage> => {
+      const sessionIds = deps.sessionManager.listSessions();
+      const sessions = sessionIds.map(id => ({
+        id,
+        messageCount: deps.sessionManager.getSession(
+          id.split(':')[0],
+          id.split(':').slice(1).join(':')
+        ).getHistory().length,
+      }));
+      return makeResponse(request.id, { sessions });
+    },
+
+    'sessions.history': async (request: GatewayRequest): Promise<OutboundMessage> => {
+      const params = request.params as { sessionId?: string; limit?: number; offset?: number } | undefined;
+      if (!params?.sessionId) {
+        return makeError(request.id, ErrorCode.InvalidRequest, 'sessionId is required');
+      }
+
+      const { sessionId, limit, offset } = params;
+      const parts = sessionId.split(':');
+      const frontend = parts[0];
+      const userId = parts.slice(1).join(':');
+      const session = deps.sessionManager.getSession(frontend, userId);
+      const allMessages = session.getHistory();
+
+      const start = offset ?? 0;
+      const end = limit ? start + limit : allMessages.length;
+      const messages = allMessages.slice(start, end);
+
+      return makeResponse(request.id, {
+        messages,
+        total: allMessages.length,
+      });
+    },
+
+    'sessions.create': async (request: GatewayRequest): Promise<OutboundMessage> => {
+      const params = request.params as { sessionId?: string } | undefined;
+      const sessionId = params?.sessionId ?? `ws:${Date.now()}`;
+      const parts = sessionId.split(':');
+      const frontend = parts[0];
+      const userId = parts.slice(1).join(':');
+
+      // Creating a session via getSession is idempotent
+      deps.sessionManager.getSession(frontend, userId);
+
+      return makeResponse(request.id, { sessionId });
+    },
+  };
+}