fix: validate health configuration before credential skips

[binaryaaron]

What changed and why

Distinguish successful, missing-credential and failed health outcomes. Validate model parameters before classifying credential skips, using the existing model conversion API.

Stack position: 2/12. Base: codex/usersim-draft-pr01-tools. Merge in stack order.

How it was verified

The unchanged candidate has a strict schema-v3 Arc acceptance. Offline health regressions cover invalid parameters before credential skips and preserved outcome handling.

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[binaryaaron]

recheck

[3mei]

@binaryaaron Good direction, but could we please resubmit it on its own from main (it's stacked on #45 now), with scripts/check_health.py, its tests, and the Makefile and workflow wiring. CONTRIBUTING already documents make health-check, so update it there rather than adding a docs/engineering/ page; the docs are being reorganized for a documentation site, and a new tree would need folding in again.