fix(ops): deploys on an unchanged snapshot tag are code-only

chooseRefreshMode now returns snapshot/skip/crawl: an unchanged tag
skips the corpus refresh entirely (pull → render → rebuild → cutover →
smoke), GH-query failures skip too, and a failed pull-snapshot keeps
serving the existing corpus instead of falling into a crawl. On-host
crawls only happen with explicit USE_CRAWL=1 (legacy USE_SNAPSHOT=0
kept) — the implicit fallback twice turned code pushes into multi-hour
syncs. Docs: runbook semantics, Linux host-package matrix
(librsvg/harfbuzz/fonttools) in self-hosting + installing pointers.