Add actions/cache@v4 to all four CI jobs. Go module cache (go/pkg/mod + go-build) keyed on go.sum for test, race, and vulns jobs. npm cache (~/.npm + node_modules) keyed on package-lock.json for frontend job. Cuts dependency download time to near-zero on cache hit. Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent) Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>