docs: document the new hut submodule

Updates CLAUDE.md (workspace layout) and the sourcehut-refresh skill to
explain that hut lives at ~xenrox, won't be discovered by the upstream
scrape, and must be bumped manually. INDEX.md timestamp bump.

init: superproject for SourceHut documentation mirror

29 upstream SourceHut repos pinned as submodules to their latest tags,
plus Claude tooling (skills, build-index script, INDEX inventory) and
phoebe-lab production patches that are not yet upstream.

Read CLAUDE.md and .claude/INDEX.md before exploring.