feat: add npm distribution wrapper for idx-cli

Add an npm wrapper (package.json + postinstall downloader) so the Rust
binary can be installed via npm/npx. Include a local smoke test, a GitHub
Release cross-compile workflow, and nodejs in the dev shell.

- package.json: idx-cli v0.2.3, bin idx, engines node>=16
- scripts/install.js: dependency-free postinstall that downloads the
  platform-specific GitHub Release asset (or IDX_BINARY_URL override)
- scripts/idx.js: binary launcher preserving exit codes/signals
- scripts/npm-smoke.sh: npm pack -> install tarball -> run idx --help
  and idx config --help, locale, no publish required
- .github/workflows/release.yml: cross-compile linux-x64/arm64 and
  darwin-arm64 releases and attach binaries to the version tag
- flake.nix: add nodejs_22 to the dev shell
- docs/NPM_DISTRIBUTION.md: usage, local test, and release flow

scripts/idx.js

@@ -0,0 +1,31 @@
+#!/usr/bin/env node
+"use strict";
+
+const fs = require("node:fs");
+const path = require("node:path");
+const { spawn } = require("node:child_process");
+
+const binaryName = process.platform === "win32" ? "idx.exe" : "idx";
+const binaryPath = path.join(__dirname, "..", "bin", binaryName);
+
+if (!fs.existsSync(binaryPath)) {
+  console.error(`idx-cli: installed binary not found at ${binaryPath}`);
+  console.error("idx-cli: rerun npm install or set IDX_BINARY_URL for a local binary");
+  process.exit(1);
+}
+
+const child = spawn(binaryPath, process.argv.slice(2), { stdio: "inherit" });
+
+child.once("error", (error) => {
+  console.error(`idx-cli: failed to start ${binaryPath}: ${error.message}`);
+  process.exit(1);
+});
+
+child.once("exit", (code, signal) => {
+  if (signal) {
+    process.kill(process.pid, signal);
+    return;
+  }
+
+  process.exit(code === null ? 1 : code);
+});