Add prepare eval gate

[ryanbaumann]

What changed

• Added a deterministic Prepare eval gate in the web app before image rendering.
• Surfaced Prepare eval pass/warn status in the Studio thought stream.
• Moved dynamic filename generation off the critical render path with a local slug fallback.
• Synced the portable skill and npm CLI to the same PrepareResult contract and eval gate; bumped skill package metadata to v3.1.0.
• Documented the audit findings and follow-up eval work in docs/agent-loop-audit.md.

Why

The agent loop already had a strong two-agent shape, but the handoff from Prepare to Render was only prompt-instructed, not locally enforced. This PR makes the handoff deterministic, keeps optional sidecars from failing successful renders, and keeps the portable skill/CLI aligned with the web app logic.

Validation

• npm run lint && npm test && npm run build
• npm run build
• python3 -m py_compile skill/infographic-agent/portable_infographic.py
• python3 skill/infographic-agent/portable_infographic.py --help
• node --check skill/infographic-agent/bin/infographic-agent.js
• node skill/infographic-agent/bin/infographic-agent.js --help
• direct/no-research Prepare eval smoke via Python import
• skill-creator quick_validate.py skill/infographic-agent with PyYAML installed in a temporary PYTHONPATH

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