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

.github/workflows/release.yml

@@ -0,0 +1,77 @@
+name: Release
+
+on:
+  push:
+    tags:
+      - "v*"
+
+permissions:
+  contents: write
+
+jobs:
+  build:
+    name: Build ${{ matrix.asset }}
+    runs-on: ${{ matrix.os }}
+    strategy:
+      fail-fast: false
+      matrix:
+        include:
+          - os: ubuntu-latest
+            target: x86_64-unknown-linux-gnu
+            asset: linux-x64
+          - os: ubuntu-latest
+            target: aarch64-unknown-linux-gnu
+            asset: linux-arm64
+          - os: macos-14
+            target: aarch64-apple-darwin
+            asset: darwin-arm64
+
+    steps:
+      - name: Check out source
+        uses: actions/checkout@v4
+
+      - name: Install Rust toolchain and target
+        uses: dtolnay/rust-toolchain@stable
+        with:
+          targets: ${{ matrix.target }}
+
+      - name: Install Linux ARM64 linker
+        if: matrix.target == 'aarch64-unknown-linux-gnu'
+        run: sudo apt-get update && sudo apt-get install --yes gcc-aarch64-linux-gnu
+
+      - name: Build release binary
+        shell: bash
+        run: |
+          if [[ "${{ matrix.target }}" == "aarch64-unknown-linux-gnu" ]]; then
+            export CARGO_TARGET_AARCH64_UNKNOWN_LINUX_GNU_LINKER=aarch64-linux-gnu-gcc
+          fi
+          cargo build --release --locked --target "${{ matrix.target }}"
+          cp "target/${{ matrix.target }}/release/idx" "idx-${{ matrix.asset }}"
+          chmod 0755 "idx-${{ matrix.asset }}"
+
+      - name: Upload target artifact
+        uses: actions/upload-artifact@v4
+        with:
+          name: idx-${{ matrix.asset }}
+          path: idx-${{ matrix.asset }}
+          if-no-files-found: error
+
+  publish:
+    name: Attach binaries to release
+    needs: build
+    runs-on: ubuntu-latest
+    steps:
+      - name: Download target artifacts
+        uses: actions/download-artifact@v4
+        with:
+          pattern: idx-*
+          merge-multiple: true
+
+      - name: Attach binaries to the version tag
+        uses: softprops/action-gh-release@v2
+        with:
+          tag_name: ${{ github.ref_name }}
+          files: |
+            idx-linux-x64
+            idx-linux-arm64
+            idx-darwin-arm64