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/vite.config.ts

@@ -2,19 +2,22 @@ import react from "@vitejs/plugin-react";
 import { defineConfig } from "vite";
 
 // https://vite.dev/config/
-export default defineConfig(() => ({
-  plugins: [react()],
-  server: {
-    port: 5173,
-    proxy: {
-      "/api": {
-        target: "http://127.0.0.1:3001",
-        timeout: 120000,
+export default defineConfig(() => {
+  const backendPort = process.env.VITE_STORYKIT_PORT || "3001";
+  return {
+    plugins: [react()],
+    server: {
+      port: 5173,
+      proxy: {
+        "/api": {
+          target: `http://127.0.0.1:${backendPort}`,
+          timeout: 120000,
+        },
       },
     },
-  },
-  build: {
-    outDir: "dist",
-    emptyOutDir: true,
-  },
-}));
+    build: {
+      outDir: "dist",
+      emptyOutDir: true,
+    },
+  };
+});