fix(npm): fail postinstall when binary download fails

[akmhatey-ai]

Summary

• Make npm postinstall fail with exit code 1 when the release binary download fails.
• Add a dependency-free regression test for an HTTP 404 download failure and run it in the npm-pack CI job.

Fixes #26.

Validation

• node --check npm/postinstall.js
• node --check npm/postinstall.test.js
• npm run test:postinstall
• npm pack --dry-run
• git diff --check
• git diff --cached --check
• git diff --cached | gitleaks detect --pipe --redact --verbose --no-color

Notes

• This is limited to the npm postinstall failure path.
• Go tests were not rerun for this npm-only change. In this Windows checkout, go test ./... is blocked by upstream vendor metadata, and go test -mod=mod ./... fails in existing cmd/optiqor golden output path-separator expectations unrelated to this change.
• Prepared with AI assistance; validation commands are listed above.

[github-actions]

First PR — welcome aboard!

A few things to expect:

1. CI: every PR runs build + race tests + lint on Ubuntu and macOS. If something fails, the log will tell you exactly which gate.
2. DCO: every commit needs Signed-off-by:. git commit -s adds it automatically.
3. Conventional Commits: PR titles like feat(analyze): new rule or fix(cli): handle X. We squash-merge by default.
4. Review: a maintainer will review within 72 hours. Suggestions are conversations, not orders — push back if something doesn't fit your context.

If you get stuck, reply here or jump to Discussions. We want this PR to land.