Add checksum validation to install script

[devm33]

SHA256SUMS.txt available in releases starting with 0.0.370-1

$ VERSION=0.0.370-1 ./install.sh
Installing GitHub Copilot CLI...
Downloading from: https://github.com/github/copilot-cli/releases/download/v0.0.370-1/copilot-darwin-arm64.tar.gz
✓ Checksum validated
Notice: Replacing copilot binary found at /Users/devm33/.local/bin/copilot.
✓ GitHub Copilot CLI installed to /Users/devm33/.local/bin/copilot

Warning: /Users/devm33/.local/bin is not in your PATH
Add it to your PATH by adding this line to your shell profile:
  export PATH="$PATH:/Users/devm33/.local/bin"

Installation complete! Run 'copilot help' to get started.

[Copilot]

Pull request overview

This PR adds SHA256 checksum validation to the install script to verify the integrity of downloaded releases. The validation occurs after downloading the tarball but before extracting it, providing an additional security layer.

Key changes:

• Downloads SHA256SUMS.txt file from the release and validates the tarball checksum
• Supports both versioned and latest release URLs
• Gracefully handles cases where checksums are unavailable or validation tools are missing

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[Copilot]

Pull request overview

Copilot reviewed 1 out of 1 changed files in this pull request and generated 2 comments.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.