feat(verify): reuse uv cache offline in sandbox

[unohee]

Summary

• Reuse the host uv artifact cache in the isolated verifier with UV_OFFLINE=1; Linux sandbox mounts host root read-only, keeping cache consumption read-only while uv provisions each scratch checkout.
• Preserve existing toolchain-path exports in a small module and test cache propagation plus offline cache-miss classification.

Verification

• Targeted Vitest: 7 passed (cache propagation, cache miss handling, and PATH regression tests).
• npm run typecheck, npm run lint, npm run build passed.
• Full runner suite cannot pass on this host: bubblewrap is blocked by AppArmor user-namespace policy; 35 tests fail at sandbox startup for that environmental reason.