core: make server runtime-agnostic by migrating from Bun to Node.js HTTP/WebSocket APIs

This enables running the opencode server on standard Node.js runtimes without requiring Bun-specific APIs. Users can now deploy the server in more environments including standard Node.js containers and cloud platforms that don't support Bun.

packages/opencode/script/build-node.ts

@@ -0,0 +1,8 @@
+#!/usr/bin/env bun
+
+Bun.build({
+  entrypoints: ["./src/node.ts"],
+  target: "node",
+  outdir: "./dist",
+  format: "esm",
+})