Story 32: Multi-Instance Worktree Support

Add configurable port via STORYKIT_PORT env var (default 3001). Server
prints machine-readable STORYKIT_PORT=<port> on startup and writes
.story_kit_port file for discovery. Frontend proxy and WebSocket read
VITE_STORYKIT_PORT env var instead of hardcoding port 3001.

7 new tests (4 backend, 3 frontend) all passing.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/api/client.ts

@@ -57,6 +57,14 @@ export interface CommandOutput {
 const DEFAULT_API_BASE = "/api";
 const DEFAULT_WS_PATH = "/ws";
 
+export function resolveWsHost(
+	isDev: boolean,
+	envPort: string | undefined,
+	locationHost: string,
+): string {
+	return isDev ? `127.0.0.1:${envPort || "3001"}` : locationHost;
+}
+
 function buildApiUrl(path: string, baseUrl = DEFAULT_API_BASE): string {
 	return `${baseUrl}${path}`;
 }
@@ -225,9 +233,11 @@ export class ChatWebSocket {
 		ChatWebSocket.refCount += 1;
 
 		const protocol = window.location.protocol === "https:" ? "wss" : "ws";
-		const wsHost = import.meta.env.DEV
-			? "127.0.0.1:3001"
-			: window.location.host;
+		const wsHost = resolveWsHost(
+			import.meta.env.DEV,
+			import.meta.env.VITE_STORYKIT_PORT,
+			window.location.host,
+		);
 		const wsUrl = `${protocol}://${wsHost}${wsPath}`;
 
 		if (