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.test.ts

@@ -1,5 +1,5 @@
 import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
-import { api } from "./client";
+import { api, resolveWsHost } from "./client";
 
 const mockFetch = vi.fn();
 
@@ -135,4 +135,22 @@ describe("api client", () => {
 			expect(result.exit_code).toBe(0);
 		});
 	});
+
+	describe("resolveWsHost", () => {
+		it("uses env port in dev mode", () => {
+			expect(resolveWsHost(true, "4200", "example.com")).toBe("127.0.0.1:4200");
+		});
+
+		it("defaults to 3001 in dev mode when no env port", () => {
+			expect(resolveWsHost(true, undefined, "example.com")).toBe(
+				"127.0.0.1:3001",
+			);
+		});
+
+		it("uses location host in production", () => {
+			expect(resolveWsHost(false, "4200", "myapp.com:8080")).toBe(
+				"myapp.com:8080",
+			);
+		});
+	});
 });