docs: add CI badges and Linux platform note

[benvinegar]

• Add CI and Integration status badges below the header
• Note that Hornet is built for Linux (kernel-level features)
• State that every PR is integration-tested on fresh Ubuntu 24.04 and Arch Linux droplets

[greptile-apps]

Greptile Summary

Added CI and Integration workflow badges to the README header and clarified that Hornet is Linux-only with kernel-level dependencies. Confirms integration testing on Ubuntu 24.04 and Arch Linux droplets for every PR.

• CI and Integration badges link to the correct GitHub Actions workflows
• Platform requirements clearly stated upfront to set proper expectations
• Integration testing claims align with the actual integration.yml workflow configuration

Confidence Score: 5/5

• This PR is safe to merge with no risk
• Documentation-only changes that accurately reflect the repository's CI infrastructure and platform requirements. No code logic, security, or functional changes.
• No files require special attention

Important Files Changed

Filename	Overview
README.md	Added CI badges and Linux platform note with integration testing details

_{Last reviewed commit: 1c83385}