feat(agent): add engine-neutral model-turn journal contract - #337

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683, actions/setup-go@f111f3307d8850f501ac008e886eec1fd1932a34, actions/setup-java@v4, actions/setup-node@v4, actions/setup-python@v5. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/

Error: Path Validation Error: Path(s) specified in the action for caching do(es) not exist, hence no cache is being saved.

Restore cache failed: Dependencies file is not found in /home/runner/work/argi/argi. Supported file pattern: go.sum