fix(claude-agent-sdk): replace the ToS-violating claude -p turn with Anthropic's own harness - #5800

PR hygiene failed: unsponsored_surface

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. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/