fix: Cursor Docker install and playground Next 16 / PORT

Summary

• Fix Cursor on Docker: node:*-slim has no curl, so the official installer failed with curl: command not found. After a successful install, spawnSync agent still missed ~/.local/bin because sandbox exec used a root-oriented PATH and HOME=/root. Setup now installs curl (and fails if apt does), sets HOME to the node user home, and puts ~/.local/bin on PATH. The Cursor runner resolves the installer symlink directly.
• Playground launch now honors --port / -p / PORT (was hardcoded to 3000). Next 16.2.4+ crashes on published playground builds that omit routes-manifest.json#onMatchHeaders (Cannot read properties of undefined (reading 'map')). The playground writes an empty array before next start, prefers the monorepo / file: playground bin, and falls back to next dev when there is no production BUILD_ID.

Context

Running Cursor evals in Docker (sandbox: 'docker') and PORT=3001 npx @vercel/agent-eval playground both failed from a file: consumer. These are the harness-side fixes so Docker Cursor installs and the playground start path work on Next 16.3.

Testing

• Unit tests: Cursor runner PATH resolution, Docker apt/PATH helpers, playground port/bin/manifest/BUILD_ID resolution, CLI --help for -p / PORT.
• Docker canary: sandbox user can run a ~/.local/bin binary and curl --version.
• Manual: PORT=3011 npx @vercel/agent-eval playground against picogrid-ui-evals — banner printed 3011, Next 16.3.4 started in next dev, and home / experiments / compare / evals / a failed Cursor run all loaded.

Issues

None.

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