feat(core-eslint): rule no-direct-socket-io

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

packages/core-eslint/rules/no-direct-socket-io.js

@@ -0,0 +1,33 @@
+// packages/core-eslint/rules/no-direct-socket-io.js
+const ALLOWED = [
+  /\/packages\/core-realtime\/src\//,
+  /\/apps\/[^/]+\/server\.ts$/,
+];
+
+export default {
+  meta: {
+    type: "problem",
+    docs: { description: "Block direct socket.io imports outside core-realtime + app servers" },
+    messages: {
+      noDirectSocketIO: 'Import from "@repo/core-realtime" instead of "socket.io". Direct imports allowed only in packages/core-realtime/src/ and apps/*/server.ts.',
+      noDirectSocketIOClient: 'Use the realtime helpers from "@repo/core-realtime" / "@repo/core-testing/instrumentation" instead of "socket.io-client".',
+    },
+    schema: [],
+  },
+  create(context) {
+    const filename = context.filename ?? context.getFilename();
+    const allowed = ALLOWED.some((re) => re.test(filename));
+    if (allowed) return {};
+
+    return {
+      ImportDeclaration(node) {
+        const source = node.source.value;
+        if (source === "socket.io") {
+          context.report({ node, messageId: "noDirectSocketIO" });
+        } else if (source === "socket.io-client") {
+          context.report({ node, messageId: "noDirectSocketIOClient" });
+        }
+      },
+    };
+  },
+};

packages/core-eslint/rules/no-direct-socket-io.test.js

@@ -0,0 +1,30 @@
+// packages/core-eslint/rules/no-direct-socket-io.test.js
+import { RuleTester } from "eslint";
+import rule from "./no-direct-socket-io.js";
+
+const tester = new RuleTester({
+  languageOptions: { ecmaVersion: 2022, sourceType: "module" },
+});
+
+tester.run("no-direct-socket-io", rule, {
+  valid: [
+    // Allowed inside core-realtime
+    { code: 'import { Server } from "socket.io";', filename: "/repo/packages/core-realtime/src/socket-io-realtime-server.ts" },
+    // Allowed in app servers
+    { code: 'import { Server } from "socket.io";', filename: "/repo/apps/web-next/server.ts" },
+    // Allowed elsewhere when not importing socket.io
+    { code: 'import { foo } from "bar";', filename: "/repo/packages/blog/src/foo.ts" },
+  ],
+  invalid: [
+    {
+      code: 'import { Server } from "socket.io";',
+      filename: "/repo/packages/blog/src/foo.ts",
+      errors: [{ messageId: "noDirectSocketIO" }],
+    },
+    {
+      code: 'import { io } from "socket.io-client";',
+      filename: "/repo/packages/blog/src/ui/Component.tsx",
+      errors: [{ messageId: "noDirectSocketIOClient" }],
+    },
+  ],
+});