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

docs/NPM_DISTRIBUTION.md

@@ -0,0 +1,55 @@
+# npm distribution
+
+The npm package `idx-cli` is a small wrapper around the native `idx` binary.
+The package keeps the JavaScript launcher and postinstall downloader. The
+downloader selects a prebuilt GitHub Release asset for the current platform and
+places it at the path used by the `idx` bin entry.
+
+Supported release assets:
+
+| npm platform | Rust target | Release asset |
+| --- | --- | --- |
+| Linux x64 | `x86_64-unknown-linux-gnu` | `idx-linux-x64` |
+| Linux arm64 | `aarch64-unknown-linux-gnu` | `idx-linux-arm64` |
+| macOS arm64 | `aarch64-apple-darwin` | `idx-darwin-arm64` |
+
+Windows and other platform combinations are out of scope. The postinstall
+script reports the supported targets when it rejects a platform.
+
+## Use the package
+
+```bash
+npm install --global idx-cli
+idx --help
+
+# Or run the package without a global install.
+npx idx-cli --help
+```
+
+The package version and the release tag must match. For example, package
+version `0.2.3` downloads assets from the `v0.2.3` GitHub Release.
+
+## Local smoke test
+
+The smoke script builds or reuses `target/release/idx`, packs the npm wrapper,
+installs the tarball into a new temporary directory, and runs the installed
+`idx` command. It uses `IDX_BINARY_URL` with a local `file:` URL, so the test
+does not need a registry publish or a network download.
+
+```bash
+nix develop -c bash -c 'scripts/npm-smoke.sh'
+nix develop -c bash -c 'npm pack --dry-run'
+```
+
+The `prepare` npm script builds the Rust release binary when npm packs the
+wrapper from this checkout. The `postinstall` script performs the release
+download for normal package installation. `IDX_BINARY_URL` accepts an HTTP(S)
+URL, a `file:` URL, or an existing local path for local testing.
+
+## GitHub Release workflow
+
+Pushing a tag that matches `v*` starts
+`.github/workflows/release.yml`. The workflow builds all three supported Rust
+targets, names the binaries with the asset names above, and attaches them to
+the matching GitHub Release. The asset names and URLs must stay aligned with
+`scripts/install.js`.