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/npm-smoke.sh

@@ -0,0 +1,74 @@
+#!/usr/bin/env bash
+set -Eeuo pipefail
+
+repo_root="$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)"
+smoke_dir="$(mktemp -d "${TMPDIR:-/tmp}/idx-cli-npm-smoke.XXXXXX")"
+tarball="${smoke_dir}/idx-cli-0.2.3.tgz"
+install_dir="${smoke_dir}/install"
+npm_cache="${smoke_dir}/npm-cache"
+
+cleanup() {
+  rm -rf "$smoke_dir"
+}
+
+on_error() {
+  local status=$?
+  echo "npm smoke: FAILED on line ${BASH_LINENO[0]} (status ${status})" >&2
+  exit "$status"
+}
+
+trap cleanup EXIT
+trap on_error ERR
+
+cd "$repo_root"
+
+release_binary="${repo_root}/target/release/idx"
+if [[ -x "$release_binary" ]]; then
+  echo "npm smoke: reusing ${release_binary}"
+else
+  echo "npm smoke: building ${release_binary}"
+  cargo build --release --locked
+fi
+
+if [[ ! -x "$release_binary" ]]; then
+  echo "npm smoke: release binary was not produced at ${release_binary}" >&2
+  exit 1
+fi
+
+echo "npm smoke: packing wrapper"
+mkdir -p "$npm_cache"
+npm_config_cache="$npm_cache" npm pack --silent --pack-destination "$smoke_dir" >/dev/null
+if [[ ! -f "$tarball" ]]; then
+  echo "npm smoke: expected tarball was not produced at ${tarball}" >&2
+  exit 1
+fi
+
+mkdir -p "$install_dir"
+binary_url="$(node -e 'const { pathToFileURL } = require("node:url"); process.stdout.write(pathToFileURL(process.argv[1]).href)' "$release_binary")"
+echo "npm smoke: installing tarball into ${install_dir}"
+npm_config_cache="$npm_cache" IDX_BINARY_URL="$binary_url" npm install \
+  --prefix "$install_dir" \
+  "$tarball" \
+  --no-save \
+  --no-package-lock \
+  --no-audit \
+  --no-fund \
+  --foreground-scripts
+
+resolved_bin="${install_dir}/node_modules/.bin/idx"
+if [[ ! -x "$resolved_bin" ]]; then
+  echo "npm smoke: npm did not create an executable bin link at ${resolved_bin}" >&2
+  exit 1
+fi
+
+help_output="${smoke_dir}/help.txt"
+echo "npm smoke: running installed idx --help"
+"$resolved_bin" --help | tee "$help_output"
+if ! rg -q "Usage: idx" "$help_output"; then
+  echo "npm smoke: --help output did not contain the expected usage line" >&2
+  exit 1
+fi
+
+echo "npm smoke: running installed idx config --help"
+"$resolved_bin" config --help
+echo "npm smoke: PASS (pack, install, --help, and config --help)"