fix: harden npm distribution release and installer

- release.yml: build static musl Linux binaries (no host glibc
  requirement), add darwin-x64, publish SHA256SUMS, allow
  workflow_dispatch for an existing tag (v0.2.3 has no binaries yet),
  and run build-only on PRs that touch the workflow or installer
- install.js: verify downloads against the release SHA256SUMS
  (IDX_BINARY_SHA256 for overrides), add a 60s request timeout, use
  stream.pipeline so truncated downloads fail, and skip the download
  inside the source checkout
- idx.js: forward SIGTERM/SIGHUP to the binary and keep the signal
  exit status
- package.json: drop the prepare script (no Rust toolchain needed to
  pack/publish)
- npm-smoke.sh: read the tarball name from npm pack, use grep over rg
- .gitignore: ignore /bin/ and *.tgz
- docs: musl targets, checksums, pnpm/bun opt-in, manual release flow

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017a5Cv5Z1NzdYKgKnTiAF4E

package.json

@@ -27,7 +27,6 @@
   ],
   "scripts": {
     "postinstall": "node scripts/install.js",
-    "prepare": "cargo build --release --locked",
     "smoke": "bash scripts/npm-smoke.sh"
   }
 }