docs: generate llms.txt from the runtime sidebar

[bartlomieju]

llms.txt was the only agent-facing index still maintained by hand, and it had
drifted: it pointed at the pre-reorganisation docs structure, so
getting_started/first_project, fundamentals/testing and
fundamentals/linting_and_formatting all resolved through redirects, subhosting
pointed at a redirect too, and nine sections added since — including the whole
Migrating from Node guide — were never indexed at all. Hand maintenance means
this happens again after the next reorganisation.

So it is now derived from the runtime sidebar, which is the one structure that
cannot go stale unnoticed, because a wrong entry visibly breaks site
navigation. Descriptions come from each page's frontmatter, condensed to a
single sentence; the three build-generated trees that have no local frontmatter
carry an explicit description instead. Only top-level sidebar items are
emitted, since the CLI and standard library subtrees alone are about 90 pages
and belong in llms-full.txt rather than a curated index.

Scope is now the open-source runtime. Deno Deploy, Deno Sandbox and Subhosting
are complementary products with their own docs trees, and three links each was
enough to dilute the index without being useful for actually working on them,
so they get a single pointer apiece and stay one hop away.

llms_txt_test.ts fails if the committed file is stale or links to a page that
no longer exists, so deno task test now catches both failure modes that
produced this drift.