Underscore-prefix colocated tests under src/pages/ so the build prerenders

Astro's file router imports every .ts under src/pages/ during static-path
collection to find getStaticPaths/endpoint handlers. The colocated
*.test.ts guards there ran their top-level `import … from 'vitest'` during
that import, which throws outside the vitest runner — surfacing as the
Cloudflare prerender server returning 500 ("Vitest failed to access its
internal state") and breaking `astro build` / `npm run deploy`.

The two .meta.test.ts files added with the recent OG work (7f1cffe,
24b64f5) joined index.phase.test.ts under src/pages/ and tipped the build
over.

Prefix the three files with `_` so Astro ignores them as routes, while
vitest's `src/**/*.test.ts` glob still discovers them. Each test reads its
sibling .astro via import.meta.url, so the in-place rename keeps every
relative path working and preserves the colocated-test convention. Record
the rule as constraint #8 in AGENTS.md so future page tests follow it.