fix: verify SHA256 checksum in npm postinstall script

[jpoehnelt]

The npm installer (install.js) downloads prebuilt binaries from GitHub Releases but never verified the .sha256 checksum files that the release workflow generates alongside each archive.

Changes:

• Download the .sha256 sidecar file after fetching the archive
• Compute SHA256 of the downloaded archive using Node.js crypto
• Compare and abort installation with a clear error on mismatch

Why: Without integrity verification, a compromised CDN/DNS or tampered GitHub Release could serve a malicious binary to npm users.

Part of supply chain hardening effort.

[changeset-bot]

🦋 Changeset detected

Latest commit: ef015ce

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@googleworkspace/cli	Patch

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[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request introduces a security hardening measure to the npm installation process. By verifying the integrity of downloaded binaries against their corresponding SHA256 checksums, the package ensures that the artifacts have not been compromised during transit or storage, significantly improving the supply chain security for users.

Highlights

• Checksum Verification: Implemented SHA256 verification for downloaded binaries in the npm postinstall script to ensure integrity.
• Security Enhancement: Added a mandatory check against the .sha256 sidecar file to prevent the installation of tampered or malicious binaries.

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[gemini-code-assist]

Code Review

This pull request adds SHA256 checksum verification for downloaded binaries in the npm post-install script to ensure file integrity. The review feedback identifies a potential memory exhaustion issue when reading large binaries into memory for hashing and recommends using a stream-based approach for more efficient and robust hash calculation.