docs: lint assets/docs for our-process words shipped to callers

`packages/cli/assets/docs/` ships to people **building with** Astryx. Material about building Astryx itself — grading rubrics, promotion gates, review evidence — keeps landing there instead of the wiki, and it is only caught if a human happens to read the diff.

[#5368](https://github.com/facebook/astryx/pull/5368) added a README with the audience rule and a routing table. This is the automated half.

## What to check

Fail on these five words appearing in `assets/docs/**`:

```
rubric readiness promotion sign-off evidence
```

**Measured, not guessed:** 0 hits across the directory on `main` today, and **56** in [#5351](https://github.com/facebook/astryx/pull/5351) — the draft that prompted this. Clean signal, nothing to migrate first.

## What NOT to check

`audit`, `checklist` and `gate` were the obvious additions and all six of their current hits are innocent — migration's "Verification Checklist", "Audit every reset stylesheet". `lab` matches `label` and `available` 148 times. Adding them makes the check noise, and a noisy check gets suppressed.

## Where

Cheapest home is a case in `packages/cli/foundation/discovery/docs-discovery.test.mjs` — it already walks the directory.

The failure message should point at the README's routing table rather than just refusing, so the author knows which wiki page the material wants.