ci(pages): build the published site from scripts/build-pages.sh

[Naruto]

The site here is the documentation and nothing else, so the published tree was already one command — build-docs.sh — and what was missing was never a build step. It was the two things around it: the toolchain a fresh clone forgets, and a server over the whole tree. zensical serve builds one locale at a time, so the header language selector could not be checked at all before a publish, and the site it serves is not the site that gets uploaded.

scripts/build-pages.sh is that, and pages.yml runs it. Same move as SSPlayerForRenPy #45 and SSPlayerForWeb; this is the shape it takes in a repository whose site is docs-only.

What changed

scripts/build-pages.sh	New. prepare-docs.sh (only when zensical is missing) → build-docs.sh, both locales, English first → report what landed → optionally serve site/. key=value options and --help like the rest of scripts/. It adds no build step of its own; the two it wraps stay the owners of the locale order and --strict.
.github/workflows/pages.yml	The build job is now build-pages.sh prepare=no serve=no. prepare-docs.sh stays a step of its own so a dependency failure reads as one in the run summary.
AGENTS.md	The paragraph for it, and the claim that build-docs.sh is what pages.yml runs — true until this PR — corrected rather than left to rot.

prepare=<auto|yes|no>  auto = only when no zensical is found; yes is what a changed
                       pin in docs/requirements.txt needs; no is for CI
venv=<dir>             passed through to prepare-docs.sh / build-docs.sh
serve=<yes|no> port=   one server over both locales — the point of the script, and
                       the one thing `zensical serve` cannot do

Verification

• A full run (scripts/build-pages.sh, no options): both locales build --strict-clean, site/index.html and site/ja/index.html reported present.
• Option parsing, --help and the validation branches (a bad enum, a non-numeric port) exercised; bash -n clean.

Not included

No .ps1 twin. The site here carries no demo, so a local Pages build is a convenience rather than the only way to see what ships, and build-docs.ps1 still covers the Windows path for the half that gates a merge.