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/install.js

@@ -0,0 +1,192 @@
+#!/usr/bin/env node
+"use strict";
+
+const fs = require("node:fs");
+const http = require("node:http");
+const https = require("node:https");
+const path = require("node:path");
+const { URL, fileURLToPath } = require("node:url");
+
+const packageRoot = path.resolve(__dirname, "..");
+const packageManifest = JSON.parse(
+  fs.readFileSync(path.join(packageRoot, "package.json"), "utf8"),
+);
+const binaryName = process.platform === "win32" ? "idx.exe" : "idx";
+const binaryDirectory = path.join(packageRoot, "bin");
+const binaryPath = path.join(binaryDirectory, binaryName);
+const githubRepository = "0xrsydn/idx-cli";
+const maxRedirects = 5;
+
+const targets = {
+  "linux/x64": {
+    label: "linux-x64",
+    triple: "x86_64-unknown-linux-gnu",
+    asset: "idx-linux-x64",
+  },
+  "linux/arm64": {
+    label: "linux-arm64",
+    triple: "aarch64-unknown-linux-gnu",
+    asset: "idx-linux-arm64",
+  },
+  "darwin/arm64": {
+    label: "darwin-arm64",
+    triple: "aarch64-apple-darwin",
+    asset: "idx-darwin-arm64",
+  },
+};
+
+function currentTarget() {
+  const key = `${process.platform}/${process.arch}`;
+  const target = targets[key];
+  if (!target) {
+    const supported = Object.values(targets)
+      .map(({ label, triple }) => `${label} (${triple})`)
+      .join(", ");
+    throw new Error(
+      `unsupported platform ${key}; supported targets are ${supported}`,
+    );
+  }
+  return target;
+}
+
+function releaseAssetUrl(target) {
+  return `https://github.com/${githubRepository}/releases/download/v${packageManifest.version}/${target.asset}`;
+}
+
+function ensureBinaryDirectory() {
+  fs.mkdirSync(binaryDirectory, { recursive: true });
+}
+
+function temporaryBinaryPath() {
+  return `${binaryPath}.${process.pid}.tmp`;
+}
+
+function replaceWithLocalBinary(sourcePath) {
+  const resolvedSource = path.resolve(sourcePath);
+  const sourceStat = fs.statSync(resolvedSource, { throwIfNoEntry: false });
+  if (!sourceStat || !sourceStat.isFile()) {
+    throw new Error(`local binary does not exist or is not a file: ${resolvedSource}`);
+  }
+
+  const temporaryPath = temporaryBinaryPath();
+  try {
+    fs.copyFileSync(resolvedSource, temporaryPath);
+    fs.chmodSync(temporaryPath, 0o755);
+    fs.renameSync(temporaryPath, binaryPath);
+  } finally {
+    if (fs.existsSync(temporaryPath)) {
+      fs.unlinkSync(temporaryPath);
+    }
+  }
+}
+
+function download(url, destination, redirects = 0) {
+  if (redirects > maxRedirects) {
+    return Promise.reject(new Error(`too many redirects while downloading ${url}`));
+  }
+
+  let parsedUrl;
+  try {
+    parsedUrl = new URL(url);
+  } catch (error) {
+    return Promise.reject(new Error(`invalid IDX_BINARY_URL: ${error.message}`));
+  }
+
+  if (!["http:", "https:"].includes(parsedUrl.protocol)) {
+    return Promise.reject(new Error(`unsupported download protocol: ${parsedUrl.protocol}`));
+  }
+
+  const client = parsedUrl.protocol === "https:" ? https : http;
+  return new Promise((resolve, reject) => {
+    const request = client.get(
+      parsedUrl,
+      {
+        headers: {
+          Accept: "application/octet-stream",
+          "User-Agent": `idx-cli-npm/${packageManifest.version}`,
+        },
+      },
+      (response) => {
+        const status = response.statusCode || 0;
+        if (status >= 300 && status < 400 && response.headers.location) {
+          const redirectedUrl = new URL(response.headers.location, parsedUrl).toString();
+          response.resume();
+          download(redirectedUrl, destination, redirects + 1).then(resolve, reject);
+          return;
+        }
+
+        if (status !== 200) {
+          response.resume();
+          reject(new Error(`download failed with HTTP ${status} for ${url}`));
+          return;
+        }
+
+        const output = fs.createWriteStream(destination, { mode: 0o755 });
+        output.once("finish", resolve);
+        output.once("error", reject);
+        response.once("error", reject);
+        response.pipe(output);
+      },
+    );
+    request.once("error", reject);
+  });
+}
+
+async function replaceWithDownloadedBinary(url) {
+  const temporaryPath = temporaryBinaryPath();
+  try {
+    await download(url, temporaryPath);
+    fs.chmodSync(temporaryPath, 0o755);
+    fs.renameSync(temporaryPath, binaryPath);
+  } finally {
+    if (fs.existsSync(temporaryPath)) {
+      fs.unlinkSync(temporaryPath);
+    }
+  }
+}
+
+function localPathFromOverride(value) {
+  if (value.startsWith("file:")) {
+    const parsedUrl = new URL(value);
+    if (parsedUrl.protocol !== "file:") {
+      throw new Error(`unsupported local URL protocol: ${parsedUrl.protocol}`);
+    }
+    return fileURLToPath(parsedUrl);
+  }
+
+  if (!/^[a-z][a-z\d+.-]*:\/\//i.test(value)) {
+    const candidate = path.resolve(value);
+    if (fs.existsSync(candidate)) {
+      return candidate;
+    }
+  }
+
+  return null;
+}
+
+async function main() {
+  const target = currentTarget();
+  const override = process.env.IDX_BINARY_URL;
+  const source = override || releaseAssetUrl(target);
+
+  ensureBinaryDirectory();
+  if (override) {
+    const localPath = localPathFromOverride(override);
+    if (localPath) {
+      replaceWithLocalBinary(localPath);
+      console.log(`idx-cli: installed local binary from ${path.resolve(localPath)}`);
+      return;
+    }
+    console.log(`idx-cli: downloading binary from IDX_BINARY_URL (${override})`);
+  } else {
+    console.log(`idx-cli: downloading ${target.asset} for ${target.label} from ${source}`);
+  }
+
+  await replaceWithDownloadedBinary(source);
+  console.log(`idx-cli: installed binary at ${binaryPath}`);
+}
+
+main().catch((error) => {
+  console.error(`idx-cli: ${error.message}`);
+  process.exit(1);
+});