Inventory: classify review-companion checks as script-now / script-later / keep-agent

## Context

Follow-on from #457. Part of the "scripts over agent checks" track: before implementing scripts, inventory what the review companion (`review-guide-pr.mdc`, `/lint`, `/check`, `/attack`) checks today and classify each.

## Scope

- List review-companion checks and classify each as:
- **Script now** — mechanical/deterministic, high-volume (e.g. IDs, required fields, duplicate titles, missing `navmenu-open`/`on-page`, fragile selector patterns, singleton multistep violations)
- **Script later** — scriptable but lower priority/ROI
- **Keep agent** — requires judgment (pedagogy, tooltip quality, journey coherence, adversarial UX)
- Note what already exists in `/lint`, `/check`, and repo tooling that could be hardened into scripts

## Deliverable

A short inventory doc mapping checks to script-now/script-later/keep-agent, to prioritize the follow-up implementation issue.

## Related

Part of epic #577. Split from #457.