Add macOS and Linux direct install script

[efrazer-oai]

Summary

• add a direct install script for macOS and Linux at scripts/install/install.sh
• stage install.sh into dist/ during release so it is published as a GitHub release asset
• reuse the existing platform npm payload so the installer includes both codex and rg

Testing

• bash -n scripts/install/install.sh
• local macOS curl | sh smoke test against a locally served copy of the script

[github-actions]

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[efrazer-oai]

I have read the CLA Document and I hereby sign the CLA

[efrazer-oai]

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