drift: installers fail against rantaiclaw v0.18.2-alpha

[github-actions]

Weekly drift run failed against rantaiclaw v0.18.2-alpha. A release bundles whatever rantaiclaw release is newest, so this is what the next tag would ship. Fix the installers, or pin the bundled version, before cutting it. Run: https://github.com/RantAI-dev/RantAI-Copilot/actions/runs/31268208874

[sulthannauval]

Fixed in #6 (feed20a7). The bundle was never at fault — 17 of 18 assertions passed and the offline install was clean. drift.yml did not install cosign, and rantaiclaw degrades to an unverified install when cosign is absent, so claw-ui signature verified could not pass on the runner and would have meant nothing if it had. cosign is now installed in the workflow and the test demands it up front. A dispatched drift run on the fix branch was green against rantaiclaw v0.18.2-alpha: installer-matrix 61/0, bundle-consistency 23/0, airgapped-bundle 18/0 including PASS claw-ui signature verified.